multi-library: hash-keyed tagged_photo + photo_insights with reconciliation

Branch B of the multi-library data-model rollout. tagged_photo and
photo_insights now follow the bytes (content_hash), not the path,
matching the policy pinned in CLAUDE.md "Multi-library data model".
Branch A's availability probe and EXIF scoping land first; this
branch builds on top.

Migration (2026-05-01-000000_hash_keyed_derived_data)

  Adds nullable content_hash columns to tagged_photo and photo_insights,
  with partial indexes on the non-null subset to keep the index small
  during the transitional window. The migration backfills from
  image_exif:
    * tagged_photo joins on rel_path alone (no library_id available);
    * photo_insights joins on (library_id, rel_path), unambiguous.
  Rows whose image_exif hash isn't known yet stay null and the runtime
  reconciliation pass populates them as the hash backlog drains.

Insert-time population

  TagDao::tag_file looks up image_exif.content_hash by rel_path before
  inserting; the hash is written into the new column.
  InsightDao::store_insight does the same scoped to (library_id,
  rel_path). Caller-supplied hash on InsertPhotoInsight wins; otherwise
  the DAO does the lookup. Both paths fall back to None if the hash
  isn't known yet — reconciliation backfills.

Reconciliation (database/reconcile.rs)

  Three idempotent passes the watcher runs once per tick after the
  per-library backfill loop:
    1. tagged_photo NULL hashes → populate from image_exif by rel_path.
    2. photo_insights NULL hashes → populate by (library_id, rel_path).
    3. photo_insights scalar merge — when multiple is_current rows
       share a content_hash, keep the earliest generated_at as
       current; demote the rest. Demoted rows keep their data so
       /insights/history is unaffected; only the "current" pointer
       narrows to one per hash.

  No filesystem dependency, so reconcile doesn't need the availability
  gate; runs every tick. Logs once when something changed, debug
  otherwise.

  Tags are set-valued under the policy (union on read, already
  DISTINCT in queries), so there is no analogous tag-collapse pass —
  duplicate (tag_id, content_hash) rows across libraries are
  harmless.

Read paths are unchanged in this branch — lookup_tags_batch's
existing rel_path-via-hash-sibling expansion still produces the
correct merge. A follow-up can simplify reads to use the new column
directly for performance.

Tests: 217 pass (212 pre-existing + 5 new in reconcile covering
NULL-fill, hash-not-yet-known no-op, library scoping on insights,
earliest-wins collapse, idempotency).

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

src/main.rs

@@ -2063,6 +2063,17 @@ fn watch_files(
                 update_media_counts(Path::new(&lib.root_path), &excluded_dirs);
             }
 
+            // Reconciliation: cross-library, so it runs once per tick
+            // outside the per-library loop. Idempotent — fast no-op when
+            // there's nothing to do. Operates on the database alone, no
+            // filesystem dependency, so it doesn't need a health gate.
+            // See database::reconcile and CLAUDE.md "Multi-library data
+            // model" for the rules.
+            {
+                let mut conn = image_api::database::connect();
+                let _ = image_api::database::reconcile::run(&mut conn);
+            }
+
             if is_full_scan {
                 last_full_scan = now;
             }