feat(configdb): add AuditLog migration with monthly partitioning and DB roles (#23)

Bundle C of the #23 M1 foundation. Creates the centralized AuditLog table
with the partition function, partition scheme, partition-aligned
non-clustered indexes, and the two access-control roles documented in
alog.md §4.

Schema:
- pf_AuditLog_Month: RANGE RIGHT, 24 monthly boundaries (Jan 2026 – Dec 2027).
- ps_AuditLog_Month: ALL TO ([PRIMARY]) — dev/test parity.
- dbo.AuditLog: created via raw SQL ON ps_AuditLog_Month(OccurredAtUtc).
  Composite clustered PK {EventId, OccurredAtUtc} (partition column must be
  part of the clustered key). 22 columns matching the EF AuditEvent model.
- 5 reconciliation/query non-clustered indexes from alog.md §4
  (Channel_Status_Occurred, CorrelationId filtered, OccurredAtUtc,
  Site_Occurred, Target_Occurred filtered) — all partition-aligned.
- UX_AuditLog_EventId: non-aligned UNIQUE on EventId alone (preserves
  InsertIfNotExistsAsync idempotency from M1-T8). Non-aligned because
  partition-aligned unique indexes require the partition column in the key,
  which would weaken to composite uniqueness; the purge story (M2/M3)
  rebuilds this index around partition switches.

Access control:
- scadalink_audit_writer: GRANT INSERT + GRANT SELECT, DENY UPDATE + DENY DELETE
  on AuditLog. The explicit DENY guarantees later db_datawriter membership
  cannot quietly re-enable mutation.
- scadalink_audit_purger: GRANT SELECT on AuditLog, GRANT ALTER on SCHEMA::dbo
  (enables ALTER PARTITION FUNCTION SWITCH and SWITCH PARTITION).

Both role definitions are idempotent (IF DATABASE_PRINCIPAL_ID IS NULL).
Down() drops in reverse dependency order with IF EXISTS guards.

Integration tests (tests/ScadaLink.ConfigurationDatabase.Tests/Migrations/):
- MsSqlMigrationFixture: connects to the running infra/mssql container (or
  the SCADALINK_MSSQL_TEST_CONN override), creates a unique per-fixture
  database, applies the migrations, drops the DB on dispose. Marks itself
  Available=false when MSSQL is unreachable so tests early-return cleanly
  on CI without the dev container.
- AddAuditLogTableMigrationTests: 8 tests covering table existence,
  partition function/scheme, partition-aligned PK, the 5 named indexes,
  both roles' grants, and a smoke test that a writer-role user receives
  SqlException with "permission" on UPDATE AuditLog.

ConfigurationDatabase tests: 142 passing -> 150 passing (8 new integration
tests). Full solution builds clean.

Package: tests project locally overrides Microsoft.Data.SqlClient to 6.1.1
(EF SqlServer 10.0.7 needs >= 6.1.1; central package version is pinned at
6.0.2 for the production ExternalSystemGateway).

tests/ScadaLink.ConfigurationDatabase.Tests/ScadaLink.ConfigurationDatabase.Tests.csproj

@@ -10,7 +10,16 @@
 
   <ItemGroup>
     <PackageReference Include="coverlet.collector" />
+    <!--
+      Bundle C migration integration tests need Microsoft.Data.SqlClient. EF
+      SqlServer 10.0.7 pulls in SqlClient >= 6.1.1, but the central package
+      version is pinned at 6.0.2 (the version the production
+      ExternalSystemGateway uses). Override the version locally for the test
+      project only; production assemblies are unaffected.
+    -->
+    <PackageReference Include="Microsoft.Data.SqlClient" VersionOverride="6.1.1" />
     <PackageReference Include="Microsoft.EntityFrameworkCore.Sqlite" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" />
     <PackageReference Include="Microsoft.AspNetCore.DataProtection" />
     <PackageReference Include="Microsoft.AspNetCore.DataProtection.EntityFrameworkCore" />
     <PackageReference Include="Microsoft.Extensions.DependencyInjection" />