refactor(api): system/jobs/modules/instances SQL moves behind interfaces

[chodeus]

Stacked on #531 (the media_cache domain split) — this diff shows both until #531 merges, then collapses to G1c alone.

What

backend/api holds zero raw SQL after this. 23 sites moved: system.py's maintenance and health queries get DbMaintenance and SystemHealth owners; job queries land on DBWorker (table-allowlist guarded); counts move to their cache owners; the stray raw sqlite3.connect VACUUM gets an honest owner.

• Bug found and fixed: /api/system/db/artwork-matches/reset counted user-locked rows (ignored=1 / user_confirmed=1) as deleted while preserving them — the pre-count's WHERE didn't match the DELETE's. The response now reports the DELETE's own rowcount, covered at both layers.
• datetime.utcnow() leaves the repo (4 sites → datetime.now(timezone.utc)): the two cutoff comparisons are proven byte-equivalent by brute force against both stored timestamp shapes (0 mismatches across 1080×16 cases); the value sites render identically (verified through the frontend's date path).
• Every ORDER BY under LIMIT carries an id tiebreaker; every moved query passed the campaign pre-screen list.

Verification

32 new tests, every one mutation-proven to redden (one test was dropped for being unable to discriminate — SQLite's index reverse-scan already returns the tied order — documented in-code); full suite green ×2 (1590 tests); ruff clean.

Flagged for later (ledgered)

media_cache.created_at cutoff separator mismatch (real pre-existing bug — same-day rows silently excluded from media_added); scheduler.py's two raw snapshot writes → SystemHealth owners; the db.worker lambda property; the cancel-endpoint TOCTOU wording.

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📥 Commits

Reviewing files that changed from the base of the PR and between 0f068df and 6821970.

📒 Files selected for processing (17)

• backend/api/instances.py
• backend/api/jobs.py
• backend/api/media_api.py
• backend/api/modules.py
• backend/api/system.py
• backend/util/database/__init__.py
• backend/util/database/collection_cache.py
• backend/util/database/maintenance.py
• backend/util/database/media_asset_matches.py
• backend/util/database/media_cache.py
• backend/util/database/media_stats.py
• backend/util/database/poster_cache.py
• backend/util/database/system_health.py
• backend/util/database/worker.py
• backend/util/notification.py
• tests/test_media_api_queries.py
• tests/test_system_queries.py

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