refactor(worker): Classify each crawled file once, then one handler per outcome #254

From the 2026-10-16 architecture review of master after #234.

Problem

The ingestion pipeline is a linear chain built for new files. Handling of content the database already knows has been bolted onto it one branch at a time:

• LivePhotoPairingMiddleware (order 15) attaches a late MOV to a stored HEIC and saves.
• IdempotencyCheckMiddleware (order 20) skips known content. With Re-link moved/renamed files and refresh LastSeenOnNas on every crawl #35 (PR feat: Re-link moved files and refresh LastSeenOnNas on every crawl (#35) #249) it also refreshes LastSeenOnNas, re-links moved rows, saves each Live Photo half separately, and catches FilePath unique violations. To stop a failed save being retried, it puts the row's old values back.
• PersistenceMiddleware (order 60) inserts, and handles the content-hash unique race with its own copy of the "is this unique violation" check.

That is three places that save, each with its own conflict handling, and a decision ("what is this file to us?") spread across several middlewares. The next issues add more branches to the same places: #250 (content changed at a known path), #33 and #34 (missing and reappearing files).

Design

Classify each crawled item once, right after hashing, then hand it to one handler per outcome.

• A classifier loads the relevant rows in one go (by content hash and by path, for both Live Photo halves) and returns a closed set of outcomes:
  • New
  • Unchanged
  • Moved
  • DuplicateCopy
  • ContentChanged (Update the row when a file's content changes at the same path #250)
  • CompletesLivePhotoPair
• New and ContentChanged continue through EXIF extraction, proxies and persistence. Every other outcome goes to a small handler that updates the row and stops the item.
• Saving and unique-violation handling live in one place, e.g. a single IsUniqueViolation(DbUpdateException, string indexName) next to MediaAssetIndexNames. No handler restores entity values to work around EF change tracking.
• Behaviour stays exactly as Re-link moved/renamed files and refresh LastSeenOnNas on every crawl #35 and fix(worker): Pair live photos whose HEIC and MOV arrive in different crawls #217 (late Live Photo pairing) define it. Their tests are the safety net and keep passing with only seam changes.

Related: #135 (declarative middleware ordering). This issue reduces how many middlewares need ordering at all.

Acceptance

• One classifier decides the outcome; no other middleware re-queries "is this content known?".
• One helper recognises unique violations by index name; the duplicated checks are gone.
• All existing pairing, idempotency, move and persistence behaviours hold, including the concurrent-consumer races.
• Update the row when a file's content changes at the same path #250 can be implemented as one new outcome plus its handler, with no new branches in other middlewares. Say so in the PR, with the planned shape.
• scripts/mutation-changed.sh at 100%.

Blocked by #35 (PR #249). Blocks #250.