fix(node): make Photos/Video/Music enrichment survive node restarts

Only thumbnail bytes were ever durable in media_cache.db — every other
derived field (photo width/height/EXIF, video ffprobe duration/dims,
audio cover art) lived solely on the in-memory GroupIndex entry, so a
node restart re-decoded every photo through Pillow, re-ran ffprobe on
every video, and re-scanned for every album cover from scratch, even
though the answers already sat in the cache.

Adds photo_meta and video_meta tables (content-only fields, keyed by
file_id) and checks them before doing the expensive work. Audio gets no
new table: mutagen reads tags and duration in one inseparable call, so
caching duration alone buys nothing — instead cover-art extraction alone
is skipped via a new skip_cover flag when a cached cover already exists.

Deliberately excluded from all three caches: anything derived from the
filename or folder path (video display_title/season/episode via guessit,
audio artist/album folder-fallback) — those must keep being recomputed
fresh so a rename/move is still correctly re-derived by the existing
_reenrich_renamed_*_entries mechanisms, instead of silently handing back
a stale parse under the new name/location.

Regression tests prove cache reuse by deleting the source file (or cover)
between two enrichment runs, and prove rename/move correctness survives
the new cache by renaming/moving to a path that never exists on disk.

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