feat(secrets): bump ZB.MOM.WW.Secrets family to 0.5.0 and wire hub fallback endpoints

Pin all five ZB.MOM.WW.Secrets* packages 0.4.1 -> 0.5.0, which brings
SecretsGrpcHubClientOptions.FallbackEndpoints and the package's internal
FailoverSecretsHubReader. A site whose GrpcHub section lists fallback
endpoints now fails a sweep over to the next central instead of stalling
on a downed primary - safe ONLY because both central nodes serve one
shared SQL secret store (scadaproj#4), so either hub answers with the
same manifest; the appsettings comments say so and warn against listing
endpoints backed by independent stores.

appsettings.json gains "FallbackEndpoints": [] with a _fallbackEndpoints
comment, and the _endpoint note's single-endpoint-stall caveat is scoped
to the empty-list case it now only applies to.

Wiring pins (red first on 0.4.1): site + Grpc + one fallback resolves
ISecretsHubReader to FailoverSecretsHubReader with the
"zb-secrets-grpc-hub:fallback:0" keyed channel present; zero fallbacks
keeps the plain GrpcSecretsHubClient and no fallback channel - the
pre-0.5.0 container shape byte-identical.

Claude-Session: https://claude.ai/code/session_014WNM4vjoVksyyBraTXSZE1

src/ZB.MOM.WW.ScadaBridge.Host/appsettings.json

@@ -30,8 +30,10 @@
       "_comment": "Read ONLY when Secrets:Replication:Enabled is true AND Mode=Grpc; inert otherwise. ONE section, both halves: CENTRAL reads BearerToken + MaxNamesPerRequest and hosts the hub on its CentralGrpcPort h2c listener (default 8083, alongside CentralControlService); a SITE reads Endpoint + BearerToken + the sweep timings and pulls. Replication is pull-only by wire contract - the proto has no write RPC - so secrets originate at central and a site cannot push. FAIL-CLOSED: unlike SqlServer mode there is no local-only fallback; a missing BearerToken (either role) or Endpoint (site) is a startup failure naming the key.",
       "_bearerToken": "Shared credential every follower presents. Supply it from appsettings or the environment (Secrets__GrpcHub__BearerToken), NOT as a ${secret:...} reference - resolving that reference is what the hub exists to make possible, so it cannot come from the hub. Same rationale and same handling as the mesh pre-shared keys. Never commit a real value here; the empty default below is fail-closed, not open.",
       "BearerToken": "",
-      "_endpoint": "SITE ONLY. Absolute http/https URI of a CENTRAL node's gRPC (h2c) port - the same address family as ScadaBridge:Communication:CentralGrpcEndpoints, e.g. 'http://central-a-host:8083'. NOT via Traefik (HTTP/1 only). NOTE the asymmetry with CentralGrpcEndpoints: that is a LIST and fails over across the central pair, whereas the hub client dials a SINGLE endpoint. A sweep against a downed central-a therefore stalls rather than failing over - which is survivable because the sweep is best-effort (one warning per interval, then retry) and the site keeps serving its full local last-known-good store, but it does mean secrets stop converging until that central node returns.",
+      "_endpoint": "SITE ONLY. Absolute http/https URI of a CENTRAL node's gRPC (h2c) port - the same address family as ScadaBridge:Communication:CentralGrpcEndpoints, e.g. 'http://central-a-host:8083'. NOT via Traefik (HTTP/1 only). With FallbackEndpoints left empty the hub client dials this SINGLE endpoint only: a sweep against a downed central-a then stalls rather than failing over - survivable because the sweep is best-effort (one warning per interval, then retry) and the site keeps serving its full local last-known-good store, but secrets stop converging until that central node returns. Listing the second central under FallbackEndpoints removes that stall.",
       "Endpoint": "",
+      "_fallbackEndpoints": "SITE ONLY. List the SECOND central node's hub endpoint here (same URI form as Endpoint, e.g. 'http://central-b-host:8083') so a sweep fails over across the central pair instead of stalling on a downed primary. Safe ONLY because both central nodes serve ONE shared SQL secret store, so either hub answers with the same manifest - NEVER list an endpoint backed by an independent store: failing over to an emptier hub is a silent convergence stop, the exact defect scadaproj#4 recorded. Empty by default; leaving it empty keeps the pre-0.5.0 single-endpoint behavior byte-identical.",
+      "FallbackEndpoints": [],
       "SyncInterval": "00:00:30",
       "SyncOnStartup": true,
       "CallDeadline": "00:00:30",