fix(photos): isolate media searches and keep optional RAW failures nonblocking

[veryCrunchy]

Parent epic: #14
Regression of: #239
Related implementation: #241

User problem

Opening Photos can fail with WebDAV media search failed even when ordinary image and video results are available. Entering the Folders destination can also start an ordinary Timeline load at the same time, causing duplicate SearchDAV chains to invalidate shared pagination carryover.

Confirmed cause

The Timeline and folder-inventory consumers currently share one account-scoped carryover generation, and the Timeline load effect is not restricted to the Timeline destination. Optional RAW enrichment also treats several RAW-only response and transport failures as fatal after the ordinary MIME partitions have already succeeded.

Acceptance criteria

• Timeline and folder inventory start only their own SearchDAV chain for the active destination.
• Timeline and folder inventory use independent account-scoped carryover generations.
• Successful ordinary image and video results remain usable when optional RAW discovery is unsupported, rejected, malformed, or unavailable.
• RAW discovery is skipped until the app has evidence that RAW files need discovery.
• MIME authentication, permission, parsing, transport, and server failures remain visible.
• RAW-capable servers and libraries containing RAW files continue to discover and page those files.
• Deterministic tests cover owner isolation, destination gating, optional RAW failures, and fatal MIME failures.
• Validate the repaired Photos flow on the isolated Android emulator before merging.

Validation scope

• Focused shared and desktop media-search tests.
• Android compilation, APK build, and visible emulator verification.
• Repository and changelog hygiene.