Update the row when a file's content changes at the same path #250

Problem

A file edited in place (re-exported, rotated and saved, metadata stripped) fails ingestion on every crawl.

• Its new bytes have a hash no row carries, so IdempotencyCheckMiddleware passes it on as new.
• PersistenceMiddleware inserts a row at the same (StorageConfigId, FilePath) as the existing one and hits IX_MediaAssets_StorageConfigId_FilePath (MediaAssetIndexNames.FilePathUnique). That violation is rethrown, so the item fails, and fails again on the next crawl.
• The old row keeps pointing at the path but describes bytes that are no longer there. Once originals are served (Implement on-demand original file serving endpoint #44), the thumbnail and the original disagree.

#35 adds a second way in: if a copy of the old content exists elsewhere, the old path still exists, so that copy is classed as a duplicate and the row stays on the wrong bytes.

Decision

Update the existing row in place. An edited photo is the same memory, so it keeps its Id and its interactions (stars, likes, hidden).

Behaviour

When the hash is unknown but an active row already holds (StorageConfigId, FilePath):

• Run the item through the pipeline as usual (EXIF, proxies). At persistence, update that row instead of inserting: new ContentHash, FileName, date fields (DateCaptured, Month, Day, Year), MediaType, Metadata, and LastSeenOnNas.
• Replace its ProxyFile rows with the newly generated ones. The new proxies live in the new content's directory (IProxyDirectoryStrategy keys by content). The old directory is orphaned; delete it after the save commits, or leave it to feat(worker): Orphaned proxy file sweeper (proxies ↔ DB reconciliation) #174, and say which.
• Keep Id, Interactions, BurstId and PairedAssetId. A Live Photo whose HEIC was edited stays paired to its MOV.
• Once the row carries the new hash, a copy of the old content elsewhere no longer finds a row by hash, so it ingests as a new asset. Moves (Re-link moved/renamed files and refresh LastSeenOnNas on every crawl #35) behave as before.

Out of scope: a soft-deleted row at the path also blocks the insert, because the FilePath index covers soft-deleted rows. That belongs to restore (#34).

Acceptance

• A file whose bytes change at the same path updates its existing row. Id and interactions are unchanged; hash, metadata, dates and proxies are new.
• The item no longer fails on later crawls.
• Old proxy files are deleted after commit or explicitly left to feat(worker): Orphaned proxy file sweeper (proxies ↔ DB reconciliation) #174, and that choice is documented in CLAUDE.md.
• Pipeline compensation on failure still deletes only this attempt's proxies, never the row's previous ones.
• Tests at the middleware seam.
• scripts/mutation-changed.sh at 100%.

Blocked by #35. Blocks #44.