feat(node): share the (path,size,mtime)->hash index cache across every group

An operator routinely shares the same physical folder into more than one
group (a music library, a Séries drive) — IndexCache used to be opened
once per group (data_dir/{group_id}/index_cache.db), so the second group
to reference an already-fully-hashed multi-terabyte folder paid the same
full content read the first one did. IndexCache itself carried no
group_id in its schema; only daemon.py's wiring did. Now one instance,
opened once at startup (data_dir/index_cache.db), shared by every group's
DirectoryIndexer.

Confirmed against a real deployment (2026-08-25/26): a group sharing an
already-indexed folder with an existing group indexes it instantly, with
zero rehashing.

Also fixes a related cross-group correctness gap found during this work:
media_cache.db (thumbnails, TMDB/MusicBrainz metadata — already node-wide,
untouched by this change) was pruned for a file the moment it left *one*
group's index, even if another group's index still held the same content
hash — forcing a redundant re-fetch/re-probe/re-thumbnail for a group that
never actually lost anything. Prune now runs only once no group's index
references the file_id any more.

Adds a node admin UI action ("Maintenance" card, prune-index-cache) to
drop cache rows that no longer belong to any group's roots — skips
anything under a root that is merely temporarily unavailable (indexer.py's
"a root that goes away freezes, never empties" rule extends to this
cache too, or a reconnected drive would pay a full rehash for no reason).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013XSohfUQQiaE77qyFLgSv3

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