fix(validation): close Theme 3 — 11 input-validation / unbounded-input findings

Each finding is a focused validation guard or upper bound at a trust boundary.
Highlights:
- Commons-015: EncryptionMetadata ctor now validates Algorithm (AES-256-GCM
  only), Kdf (PBKDF2-SHA256 only), Iterations ([100k, 10M]), non-null Salt/IV.
- Transport-004: new BundleUnlockRateLimiter (sliding-window, per-key,
  singleton) wired into BundleImporter.LoadAsync; over-budget callers see
  BundleUnlockRateLimitedException. Per-bundle 3-strike + per-window cap.
- ESG-022: ExternalSystemClient.InvokeHttpAsync allow-lists the documented
  GET/POST/PUT/PATCH/DELETE set (case-insensitive); unknown verbs throw.
- SEL-015: SiteEventLogger queue now bounded (10k cap, DropOldest); dropped
  events fault their Task and increment FailedWriteCount so the drop is
  observable instead of an unbounded memory growth.
- SEL-017: EventLogQueryService clamps caller-supplied PageSize to a new
  MaxQueryPageSize cap (default 500) so int.MaxValue can't OOM the host.
- SEL-020: LogEventAsync rejects severities outside {Info, Warning, Error}
  (matches SQLite BINARY-collation query filter).
- InboundAPI-020: ContentType "json" check now case-insensitive
  (application/JSON no longer slips through as not-json).
- InboundAPI-024: _knownBadMethods capped at 1000 entries (drops new entries
  once full); per-request DB lookup remains the correctness path.
- SR-025: HandleSetStaticAttribute validates the attribute name against the
  deployed config; unknown names now return Success=false instead of
  leaking orphan override rows into the SQLite store.
- TE-021: MoveTemplateAsync runs the sibling-name-collision check at the
  destination, mirroring TemplateFolderService.MoveFolderAsync.
- TE-022: LockEnforcer's once-locked-stays-locked rule now also covers
  LockedInDerived (was previously only IsLocked).

New regression tests across 8 test projects (EncryptionMetadata, rate
limiter, ESG client allow-list, SEL bounded channel / PageSize clamp /
severity validation, InboundAPI ContentType + bad-methods cap, SiteRT
unknown-attribute, TemplateEngine MoveTemplate + LockedInDerived).
Build clean; affected suites all green. README regenerated: 93 open (was 104).

Note: a separate manual re-run was needed for the SiteEventLogging hunk
because its initial subagent's source edits never landed on disk despite
reporting success (file-collision-style failure mode).

tests/ScadaLink.SiteEventLogging.Tests/SiteEventLoggerTests.cs

@@ -140,4 +140,34 @@ public class SiteEventLoggerTests : IDisposable
         var count = (long)cmd.ExecuteScalar()!;
         Assert.Equal(6, count);
     }
+
+    // --- SiteEventLogging-020: severity validation against the closed set ---
+
+    [Theory]
+    [InlineData("info")]      // wrong casing
+    [InlineData("warn")]      // abbreviation
+    [InlineData("ERROR")]     // wrong casing
+    [InlineData("Debug")]     // not in set
+    [InlineData("Critical")]  // not in set
+    public async Task LogEventAsync_ThrowsOnUnknownSeverity(string badSeverity)
+    {
+        var ex = await Assert.ThrowsAsync<ArgumentException>(
+            () => _logger.LogEventAsync("script", badSeverity, null, "Source", "Message"));
+        Assert.Contains(badSeverity, ex.Message);
+        Assert.Contains("Info, Warning, Error", ex.Message);
+    }
+
+    [Theory]
+    [InlineData("Info")]
+    [InlineData("Warning")]
+    [InlineData("Error")]
+    public async Task LogEventAsync_AcceptsAllDocumentedSeverities(string severity)
+    {
+        await _logger.LogEventAsync("script", severity, null, "Source", "Message");
+
+        using var cmd = _verifyConnection.CreateCommand();
+        cmd.CommandText = "SELECT severity FROM site_events";
+        var stored = (string)cmd.ExecuteScalar()!;
+        Assert.Equal(severity, stored);
+    }
 }