multi-library: per-library excluded_dirs

Adds a nullable comma-separated TEXT column to the libraries table.
Effective excludes for a walk = (env-var globals) ∪
(library.excluded_dirs). Empty / NULL = no library-specific
extras; the global env var still applies.

Migration (2026-05-01-110000_libraries_excluded_dirs)

  ALTER TABLE libraries ADD COLUMN excluded_dirs TEXT. NULL on every
  existing row — no behavior change on upgrade.

Library struct + helpers (libraries.rs)

  - Library gains excluded_dirs: Vec<String>, parsed from the column
    by parse_excluded_dirs_column (drops empties / whitespace,
    matches the env-var parser).
  - Library::effective_excluded_dirs(globals) returns the union.
  - From<LibraryRow> hydrates the field on AppState construction so
    /libraries surfaces it.

Watcher / walkers / memories

  Every per-library walker now consults the effective set:
    - process_new_files (file-watch ingest, RAW/EXIF/face)
    - process_face_backlog (filter_excluded inherits)
    - create_thumbnails (startup + new-file branch)
    - update_media_counts (Prometheus gauge)
    - cleanup_orphaned_playlists (per-library source-existence check)
    - memories endpoint (PathExcluder)

  Effective set is computed once per per-library iteration in the
  watcher tick and threaded through; called functions retain their
  flat &[String] signature (no per-library awareness needed inside
  the walker primitives).

Use case: mount a parent directory while a sibling library covers
a child subtree, and exclude the child subtree from the parent so
the libraries don't double-walk / double-write image_exif. With
hash-keyed derived data (Branches B/C), the duplication-avoidance
is the only cost prevented — face / tag / insight sharing was
already correct via content_hash.

Tests: 228 pass (226 from previous + 2 new in libraries::tests:
parse_excluded_dirs_column edge cases,
effective_excluded_dirs_unions_global_and_per_library).

CLAUDE.md gains a "Per-library excludes" subsection of the
multi-library data model.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

migrations/2026-05-01-110000_libraries_excluded_dirs/down.sql

@@ -0,0 +1,2 @@
+-- Requires SQLite 3.35+ for ALTER TABLE DROP COLUMN.
+ALTER TABLE libraries DROP COLUMN excluded_dirs;