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).

code-reviews/Commons/findings.md

@@ -8,7 +8,7 @@
 | Last reviewed | 2026-05-28 |
 | Reviewer | claude-agent |
 | Commit reviewed | `1eb6e97` |
-| Open findings | 8 |
+| Open findings | 7 |
 
 ## Summary
 
@@ -732,9 +732,11 @@ describe the corrupt-typed-row branch. Regression tests added in
 |--|--|
 | Severity | Medium |
 | Category | Correctness & logic bugs |
-| Status | Open |
+| Status | Resolved |
 | Location | `src/ScadaLink.Commons/Types/Transport/EncryptionMetadata.cs:3-8` |
 
+**Resolution (2026-05-28):** Converted `EncryptionMetadata` to a non-positional `record` with an explicit constructor that enforces invariants at the type boundary: `Algorithm` must equal `"AES-256-GCM"`, `Kdf` must equal `"PBKDF2-SHA256"`, `Iterations` must lie in `[MinPbkdf2Iterations=100_000, MaxPbkdf2Iterations=10_000_000]`, and `SaltB64`/`IvB64` must be non-null (empty permitted for the BundleSerializer.Pack seed pattern). Invalid values throw `ArgumentException` (or `ArgumentNullException`) naming the offending field. Added `EncryptionMetadataTests` covering valid construction, unknown algorithm/KDF (including case sensitivity), out-of-range iteration counts (including both boundaries), and null salt/IV. Updated `BundleSecretEncryptorTests` / `BundleSerializerTests` to use `MinPbkdf2Iterations` instead of the prior 10_000 placeholder.
+
 **Description**
 
 `EncryptionMetadata` is a positional record that carries the bundle's encryption parameters