fix(siteeventlog)+fix(host): purge startup yield + auto_vacuum reality (arch-review 2026-08-19 P4-5 + P4-6)

P4-5: EventLogPurgeService.ExecuteAsync now yields (Task.Yield()) before running the
initial RunPurge(), making "the purge never runs synchronously on the caller's thread"
a property of the method itself rather than an artifact of .NET 8+'s BackgroundService
scheduling (verified empirically not to reproduce the literal "StartAsync blocks" symptom
on this repo's net10.0 target — a 1.5s synchronous Thread.Sleep in ExecuteAsync still lets
StartAsync return in ~2ms). Proven instead via a reflection-invoked ExecuteAsync test that
bypasses BackgroundService's Task.Run wrapper, which is timing-independent. Also: a
throwing isActive predicate now fails CLOSED (logged as inactive, at Warning) instead of
silently claiming "active" — the predicate only affects the tick's log line since P4-4,
but a throw must not masquerade as a successful check.

P4-6: PRAGMA auto_vacuum = INCREMENTAL only takes effect on a table-less database, and
LocalDbSchema.EnsureCreated (inside SqliteLocalDb's constructor) always creates LocalDb's
own bookkeeping tables before SiteEventLogSchema.Apply ever gets a connection — so the
pragma there had never once taken effect, in any real call path. Real fix per plan
decision 2: SiteLocalDbSetup.EnsureAutoVacuumOnFreshFile pre-creates a genuinely fresh
LocalDb file with the pragma set, called from SiteServiceRegistration.Configure BEFORE
AddZbLocalDb's lazy singleton ever constructs SqliteLocalDb — no LocalDb API change
needed. Only helps NEW site databases; an existing file (every node on the docker rig
today) is left untouched rather than VACUUM'd, per the plan's explicit rejection of an
unconditional VACUUM against a live ~1 GB file. The now-dead pragma call in
SiteEventLogSchema.Apply is removed and its comment corrected.

Tests proven to fail against the unfixed code (git-diff-and-restore, not stash — this repo
shares one stash ref across all worktrees): ExecuteAsync_YieldsBeforeRunningTheInitial-
PurgeSynchronously and RunPurge_ActiveNodeCheckThrows_TickLogsAsInactive_NotActive fail
without the EventLogPurgeService fix; SiteLocalDbAutoVacuumTests fails to compile without
EnsureAutoVacuumOnFreshFile. SiteEventLogging.Tests 112/112 and Host.Tests 526/526 green
with the fix applied.

src/ZB.MOM.WW.ScadaBridge.SiteEventLogging/SiteEventLogSchema.cs

@@ -36,15 +36,24 @@ public static class SiteEventLogSchema
     {
         ArgumentNullException.ThrowIfNull(connection);
 
-        // auto_vacuum must be set before any table is created for it to take effect
-        // on a fresh database. With INCREMENTAL mode, PRAGMA incremental_vacuum can
-        // later reclaim free pages so the storage-cap purge can shrink the file.
-        using (var pragmaCmd = connection.CreateCommand())
-        {
-            pragmaCmd.CommandText = "PRAGMA auto_vacuum = INCREMENTAL";
-            pragmaCmd.ExecuteNonQuery();
-        }
-
+        // arch-review 2026-08-19 P4-6: this method used to also set
+        // "PRAGMA auto_vacuum = INCREMENTAL" here, on the theory that setting it right
+        // before CREATE TABLE would make it take effect on a fresh database. That was
+        // wrong in every real call path: SQLite only honors an auto_vacuum change on a
+        // database that has NO tables yet, and by the time this method runs — via the
+        // Host's AddZbLocalDb onReady callback (SiteLocalDbSetup.OnReady) OR via
+        // SiteEventLogger's own InitializeSchema fallback for directly-constructed
+        // instances — the consolidated file already holds LocalDb's own bookkeeping
+        // tables (SqliteLocalDb's constructor runs LocalDbSchema.EnsureCreated on its
+        // master connection before either caller ever gets a connection). So the pragma
+        // here had never once taken effect; it was a silent, permanent no-op. The real
+        // fix now lives in Host/SiteLocalDbSetup.cs
+        // (EnsureAutoVacuumOnFreshFile), which pre-creates a genuinely fresh LocalDb file
+        // with the pragma set BEFORE AddZbLocalDb's lazy singleton ever constructs
+        // SqliteLocalDb — and, being a Host-layer, one-time, pre-file-existence step, it
+        // helps ONLY brand-new site databases; every file created before this fix shipped
+        // stays auto_vacuum=NONE forever (see the PurgeByStorageCap remarks in
+        // EventLogPurgeService for the operational consequence).
         using var cmd = connection.CreateCommand();
         cmd.CommandText = """
             CREATE TABLE IF NOT EXISTS site_events (