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>

migrations/2026-04-30-000000_unique_tag_name/down.sql

@@ -0,0 +1 @@
+DROP INDEX IF EXISTS idx_tags_name_nocase;

migrations/2026-04-30-000000_unique_tag_name/up.sql

@@ -0,0 +1,28 @@
+-- Tags only enforced uniqueness in application code (the add_tag handler
+-- looks up by name before inserting). The schema itself accepted dupes,
+-- so a divergent code path could land two tags with the same name. Now
+-- that we expose a rename endpoint we want a hard guarantee: case-
+-- insensitive UNIQUE on tags.name.
+
+-- Pre-flight: collapse exact-name duplicates (case-insensitive) onto the
+-- lowest-id row before adding the constraint, otherwise the index
+-- creation fails on any DB that ever produced dupes. On a clean DB this
+-- is a no-op.
+UPDATE tagged_photo
+SET tag_id = (
+    SELECT MIN(t2.id) FROM tags t2
+    WHERE LOWER(t2.name) = LOWER((SELECT name FROM tags WHERE id = tagged_photo.tag_id))
+)
+WHERE tag_id IN (
+    SELECT t.id FROM tags t
+    WHERE t.id <> (
+        SELECT MIN(t2.id) FROM tags t2 WHERE LOWER(t2.name) = LOWER(t.name)
+    )
+);
+
+DELETE FROM tags
+WHERE id <> (
+    SELECT MIN(t2.id) FROM tags t2 WHERE LOWER(t2.name) = LOWER(tags.name)
+);
+
+CREATE UNIQUE INDEX idx_tags_name_nocase ON tags (name COLLATE NOCASE);