tags: add edit + delete endpoints, enable FK enforcement

PUT /image/tags/{id} renames a tag globally; DELETE /image/tags/{id}
removes a tag and every photo's reference. Rename returns 200/404/409
(case-insensitive name conflict) / 400 (empty name); delete returns
204/404. New migration adds a UNIQUE COLLATE NOCASE index on
tags.name with a pre-flight pass that collapses existing case-
insensitive duplicates onto the lowest id.

The connection setup now sets PRAGMA foreign_keys = ON. The schema
already declares ON DELETE CASCADE / SET NULL on several tables —
those clauses were documentation-only because SQLite has FK
enforcement off per-connection by default. Audited every
diesel::delete site; each touches either no inbound FKs or has a
matching policy. delete_tag relies on the tagged_photo cascade
instead of doing manual cleanup.

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

src/database/mod.rs

@@ -136,10 +136,19 @@ pub fn connect() -> SqliteConnection {
     // rollback-journal durability; we accept the narrow last-fsync
     // window for the 2–10× write throughput).
     use diesel::connection::SimpleConnection;
+    // foreign_keys = ON is per-connection in SQLite (off by default), so
+    // it has to be set here alongside the other pragmas. Without it
+    // every `REFERENCES … ON DELETE CASCADE / SET NULL` clause in the
+    // schema is documentation-only — orphan rows would survive the
+    // referenced row's deletion. With it, the cascade fires
+    // automatically and code that previously did manual two-step
+    // cleanup (delete child rows, then parent) becomes redundant but
+    // still correct.
     conn.batch_execute(
         "PRAGMA journal_mode = WAL; \
          PRAGMA busy_timeout = 5000; \
-         PRAGMA synchronous = NORMAL;",
+         PRAGMA synchronous = NORMAL; \
+         PRAGMA foreign_keys = ON;",
     )
     .expect("set sqlite pragmas");
     conn