fix(localdb): phase-2 live gate — 4 production defects found and fixed

Gate record: docs/plans/2026-07-20-localdb-phase2-live-gate.md.
Checks 1, 2, 5, 6 pass. Checks 3 and 4 are NOT satisfied — the defect they
were meant to confirm turned out to be the opposite of what the plan assumed.

Three defects crash-looped every driver node before check 1 could even run:

1. An empty ServerHistorian:ApiKey kills the host. ServerHistorianOptions-
   Validator exists to turn exactly that class of failure into a named
   OptionsValidationException, but its documented fail tier explicitly excluded
   ApiKey on the reasoning that a keyless client "degrades — the gateway rejects
   calls". It does not: the client validates its own options at construction, so
   the process dies during Akka startup and never makes a call.

2. UseTls disagreeing with the endpoint scheme kills the host too, in both
   directions (both messages confirmed in the shipped client assembly). Moving an
   endpoint from https to http without clearing UseTls is an ordinary migration
   slip.

3. Plaintext h2c was UNREACHABLE. HistorianGatewayClientAdapter forwarded the
   TLS-only options unconditionally, and AllowUntrustedServerCertificate defaults
   to false, so it always sent RequireCertificateValidation=true — which the
   client rejects outright when UseTls=false. Every http:// deployment crashed,
   though the scheme is documented as the supported way to select h2c, and the
   only workaround was to assert a certificate posture for a connection that has
   no certificate.

The fourth was the blocker, and it is Phase 2's own:

4. The drain gate deferred to a Primary that cannot deliver. Redundancy roles are
   elected CLUSTER-WIDE; the alarm queue is PAIR-LOCAL. On the rig the elected
   driver Primary is central-1 — it carries the driver Akka role, replicates
   nobody's LocalDb and does not even run the alarm historian — so every driver
   node logged "Historian drain suspended", including the two site-b nodes that
   have no peer at all. Nothing drained anywhere, where before Phase 2 it drained
   fine. The cost is not a duplicate; it is the buffer growing to the capacity
   wall and evicting the audit trail it exists to protect.

   Fixed in three layers: a separate ShouldDrainAlarmHistory policy (unknown role
   drains; the two gates now deliberately disagree, and a test pins that); peer-
   host matching in DriverHostActor so a node stands down only for a Primary
   holding its rows; and AddAlarmHistorian short-circuiting the gate when
   replication is unconfigured — testing BOTH Replication:PeerAddress and
   SyncListenPort, since only the dialing half sets the former while both halves
   share the queue.

   Every one of these follows from the asymmetry: a false allow costs a duplicate
   row, which at-least-once delivery already accepts and payload-hash ids
   collapse; a false deny loses data silently.

A third vacuous test, caught by the same delete-the-guard discipline: the
role-view tests stayed green with the guard removed, because AwaitAssert polls
until an assertion passes and the assertion was "reads open" — which is the
SEEDED value, satisfied at the first poll before the actor processed anything.
They now assert the sequence of published values through a recording view; the
control then goes red for exactly the cases that matter.

Migration evidence: 11 legacy rows across two deliberately overlapping files
converged to exactly 9 identical rows on both nodes, proving D-6's payload-hash
identity on real nodes rather than in a fixture.

Open design fork, recorded in the gate doc rather than decided here: a pair
cannot currently identify its own Primary, so both halves drain. Safe in every
topology — nothing loses data — but the gate's de-duplication benefit is
unrealised until roles are scoped per pair.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

src/Server/ZB.MOM.WW.OtOpcUa.Host/Configuration/ServerHistorianOptionsValidator.cs

@@ -24,12 +24,25 @@ namespace ZB.MOM.WW.OtOpcUa.Host.Configuration;
 ///         <see cref="ServerHistorianOptions.Enabled"/> in the Host, so <c>Enabled</c> covers it.
 ///     </para>
 ///     <para>
-///         <b>Fail tier = provably-crashing configs only.</b> Only an empty / non-absolute / non-http(s)
-///         <c>Endpoint</c> fails here (it throws in the factory). Empty <c>ApiKey</c> and non-positive
-///         <c>MaxTieClusterOverfetch</c> degrade rather than crash (the gateway rejects calls / the node
-///         manager surfaces a Bad read), so they stay operator warnings in
+///         <b>Fail tier = provably-crashing configs only.</b> Three settings qualify, and they all fail
+///         the same way — <c>HistorianGatewayClientOptions.Validate()</c> throws inside the client
+///         factory before any call is attempted, so the host dies at startup:
+///         an empty / non-absolute / non-http(s) <c>Endpoint</c>; an empty <c>ApiKey</c> ("The gateway
+///         API key must not be empty"); and a <c>UseTls</c> that disagrees with the endpoint scheme
+///         ("UseTls requires an https gateway endpoint" / "An https gateway endpoint requires UseTls").
+///         A non-positive <c>MaxTieClusterOverfetch</c> genuinely degrades rather than crashes (the node
+///         manager surfaces a Bad read), so it stays an operator warning in
 ///         <see cref="ServerHistorianOptions.Validate"/>. The <c>Endpoint</c> value is not a secret and
-///         is echoed to make the error actionable; the <c>ApiKey</c> is never surfaced.
+///         is echoed to make each error actionable; the <c>ApiKey</c> is never surfaced.
+///     </para>
+///     <para>
+///         <b>The <c>ApiKey</c> and <c>UseTls</c> checks were added after two consecutive live-rig
+///         crash-loops</b> (LocalDb Phase 2 gate). Both had been classified as degrading, on the
+///         assumption that a bad client configuration would connect and be rejected by the gateway — but
+///         the client validates its own options at construction, so the process dies during Akka startup
+///         instead, which is precisely the failure this validator exists to convert into a named,
+///         aggregated error. The lesson generalises: "degrades" is only true of settings the client
+///         does not itself validate.
 ///     </para>
 /// </remarks>
 public sealed class ServerHistorianOptionsValidator : OptionsValidatorBase<ServerHistorianOptions>
@@ -62,5 +75,26 @@ public sealed class ServerHistorianOptionsValidator : OptionsValidatorBase<Serve
         builder.RequireThat(
             endpointValid,
             $"ServerHistorian:Endpoint is empty or not an absolute http(s) URI ('{options.Endpoint}') — {reason}.");
+
+        // Deliberately not echoed: unlike the endpoint, the key is a secret.
+        builder.RequireThat(
+            !string.IsNullOrWhiteSpace(options.ApiKey),
+            $"ServerHistorian:ApiKey is empty — {reason}. The gateway client rejects a keyless "
+            + "configuration at construction, so the host would crash on startup rather than degrade. "
+            + "Supply it via the environment variable ServerHistorian__ApiKey.");
+
+        // The flag and the scheme must agree in BOTH directions — the client throws either way
+        // ("UseTls requires an https gateway endpoint" / "An https gateway endpoint requires UseTls").
+        // Only meaningful once the endpoint parsed; a malformed one already failed above.
+        if (endpointValid)
+        {
+            var https = uri!.Scheme == Uri.UriSchemeHttps;
+            builder.RequireThat(
+                options.UseTls == https,
+                $"ServerHistorian:UseTls is {options.UseTls.ToString().ToLowerInvariant()} but the "
+                + $"endpoint is '{options.Endpoint}' — {reason}. The flag must match the scheme "
+                + "(https ⇒ UseTls=true, http ⇒ UseTls=false); the gateway client rejects a mismatch "
+                + "at construction, crashing the host on startup.");
+        }
     }
 }