multi-library: operator kill switch via libraries.enabled

A small follow-up to Branches A/B/C. Adds a nullable-default-1
boolean column to the `libraries` table that controls whether the
watcher considers the library at all. Useful for staging a new
mount before committing to ingest, and as a maintenance kill
switch when a library needs to be quiet without being unmounted.

Migration (2026-05-01-100000_libraries_enabled_flag)

  ALTER TABLE libraries ADD COLUMN enabled BOOLEAN NOT NULL DEFAULT 1.
  Existing rows stay enabled — no behavior change on upgrade.

Watcher gate (main.rs)

  At the top of the per-library loop, if !lib.enabled { continue; }
  — runs BEFORE the availability probe. Disabled libraries don't
  enter the health map, don't get probed, don't get ingest, don't
  get any maintenance pass. The initial sweep before the loop's
  first sleep also skips disabled libraries.

Orphan-GC consensus (library_maintenance.rs)

  all_libraries_online filters disabled libraries out of the
  consensus check — they're treated as out-of-scope, not as
  blockers. Otherwise flipping enabled=false would permanently
  halt orphan GC for the rest of the system, which is the opposite
  of the intended kill-switch semantics.

Cross-library duplicates: safe by construction. Hash-keyed derived
data (face_detections, tagged_photo with hash, photo_insights with
hash) is anchored by ANY image_exif row carrying the hash. Disabling
a library does NOT delete its image_exif rows, so a hash referenced
by a disabled library's row stays anchored — derived data survives.
collect_orphan_hashes deliberately doesn't filter image_exif by
library.enabled for exactly this reason.

No HTTP endpoint. Library mutation is rare-enough infra work that a
SQL toggle is fine, and a public mutation endpoint without a role /
permission story would be poorly-prioritized exposure for a
single-user tool. Documented in CLAUDE.md.

Tests: 226 pass (225 from Branch C + 1 new
all_libraries_online_treats_disabled_as_out_of_scope, which proves
that even an explicit Stale entry on a disabled library doesn't
block the consensus).

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

CLAUDE.md

@@ -195,6 +195,21 @@ photos that have a copy under lib N, but the derived data attached to
 those photos is the merged hash-keyed view. This is the answer to "show
 me archive photos with their original tags."
 
+**Operator kill switch (`libraries.enabled`).** Setting `enabled=0` on a
+library is a hard pause: the watcher skips it entirely — before the
+probe, before ingest, before any maintenance pass — and the orphan-GC
+all-online consensus check filters disabled libraries out (they don't
+keep the GC window closed). Reads / serving are unaffected; nothing
+prevents `/image?path=...` from resolving against a disabled library's
+root if the file is on disk. The existing `image_exif` rows for a
+disabled library are **not deleted** — they continue to anchor
+hash-keyed derived data, so cross-library duplicates survive the
+disable. Toggle via SQL; there is intentionally no HTTP endpoint for
+library mutation (single-user tool, no role / permission story).
+Typical workflows: stage a new mount with `enabled=0` then flip to `1`;
+quiet a flaky NAS during maintenance without disturbing the rest of
+the system.
+
 **Library availability and safety.** Libraries can be on network shares
 or removable media; the file watcher must not interpret a temporary
 unavailability as a mass-deletion event. Every tick begins with a