[APIView] WI-3: Backfill Revisions & Samples

## Goal

Every `APIRevisionListItemModel` and `SamplesRevisionModel` in the database has a non-null `APIVersionId`.

**Area:** Foundation · **Depends on:** WI-1, WI-2 · **Stable alone:** ✅ Yes — backfills FK on historical data; nothing reads it yet

## Deliverables

### Backfill revisions

One-time migration (console command or admin-triggered background job):
1. Fetch all non-deleted revisions where `APIVersionId == null` and `APIRevisionType != PullRequest`
2. Compute `NormalizeVersion(revision.Files[0].PackageVersion)` for each
3. Group by `(reviewId, versionIdentifier)`
4. For each group: find-or-create `APIVersionModel`, set `APIVersionId`, copy approval/release state from revisions to version
5. Handle versionless revisions (JavaScript) per §7.3: label extraction → content-hash grouping → synthetic version

**Idempotent:** Skips revisions where `APIVersionId` is already set. Can be paused and resumed.

### Backfill samples revisions

For each `SamplesRevisionModel` where `APIVersionId == null`: find closest-by-date revision in same review, adopt its `APIVersionId`. Orphaned samples fall back to latest stable version.

### Validation gate

Before downstream WIs (WI-5, WI-7, WI-8, etc.) are flag-flipped, verify:
- `SELECT COUNT(1) FROM c WHERE c.APIVersionId = null AND c.IsDeleted != true` returns 0 for both containers (excluding PR revisions)
- Every `APIVersion` has at least one linked revision

### Tests

- `BackfillMigrationTests.cs` — Grouping by normalized version; approval/release state copied; versionless handling; idempotent re-run
- `SamplesRevisionsManagerTests.cs` — Backfill assigns correct version; orphaned sample fallback

### Documentation

Update `docs/overview.md` §4c to add `APIVersionsManager`. Update `docs/release_approval.md` §5 re: approval migration to version-level.

Start by reading the named BackfillMigrationTests.cs and SamplesRevisionsManagerTests.cs alongside the APIRevisionListItemModel and SamplesRevisionModel requirements, then review the referenced docs/overview.md §4c and docs/release_approval.md §5. Run the backfill and validation queries against representative historical data; done means both backfills are idempotent, versionless and orphaned samples are handled, and the stated validation checks pass.

Written by the indexing model from the issue text.