feat(page-tags): normalize page_tags onto the shared tags table

page_tags previously stored the tag inline as free-form text — the lone
un-normalized tag concept, while authors/genres/manga-tags all went
through a lookup table + FK. Fold page tags into the SAME shared `tags`
table that manga_tags uses: `page_tags.tag` becomes `page_tags.tag_id`
referencing `tags(id)`. Manga tags and page tags now share one global
vocabulary.

The HTTP contract is unchanged — the API still speaks tag *names*; the
repo resolves name<->id via `repo::tag::upsert_by_name` (the manga-tag
helper) and keeps the stricter page-tag `normalize_tag` at the boundary.
Frontend, DTOs, and response shapes are untouched. User-visible effect:
page-tag names now appear in the manga-tag autocomplete and vice versa.

Migration 0024 backfills `tags` from existing inline values (deduping
case-insensitively), repoints rows, swaps the unique/secondary indexes
to tag_id, and drops the inline column.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/migrations/0024_normalize_page_tags.sql

@@ -0,0 +1,48 @@
+-- Normalize per-page tags. Originally (0023) `page_tags.tag` stored the
+-- tag inline as free-form text, the lone un-normalized tag concept while
+-- authors/genres/manga-tags all went through a lookup table + FK. This
+-- migration folds page tags into the SAME shared `tags` table that
+-- `manga_tags` uses (0009): `page_tags.tag` becomes `page_tags.tag_id`
+-- referencing `tags(id)`. After this there is one global tag vocabulary
+-- shared by manga tags and page tags.
+--
+-- The HTTP contract is unchanged — the API still speaks tag *names*; the
+-- repo resolves name<->id via `repo::tag::upsert_by_name`, exactly as the
+-- manga-tag path does.
+
+-- 1. New FK column, nullable until backfilled.
+ALTER TABLE page_tags
+    ADD COLUMN tag_id uuid REFERENCES tags(id) ON DELETE CASCADE;
+
+-- 2. Seed the lookup table from the existing inline values, deduping
+--    case-insensitively via the tags (lower(name)) unique index. Mirrors
+--    the author backfill in 0009.
+INSERT INTO tags (name)
+    SELECT DISTINCT tag FROM page_tags
+ON CONFLICT (lower(name)) DO NOTHING;
+
+-- 3. Point every page_tags row at its canonical tag row.
+UPDATE page_tags pt
+   SET tag_id = t.id
+  FROM tags t
+ WHERE lower(t.name) = lower(pt.tag);
+
+-- 4. The FK is now mandatory.
+ALTER TABLE page_tags ALTER COLUMN tag_id SET NOT NULL;
+
+-- 5. Swap the inline-tag indexes/constraints for tag_id equivalents.
+DROP INDEX IF EXISTS page_tags_user_page_tag_uniq;
+CREATE UNIQUE INDEX page_tags_user_page_tag_uniq
+    ON page_tags (user_id, page_id, tag_id);
+
+DROP INDEX IF EXISTS page_tags_user_tag_idx;
+CREATE INDEX page_tags_user_tag_idx
+    ON page_tags (user_id, tag_id, created_at DESC);
+
+-- page_tags_user_idx (user_id, created_at DESC) and page_tags_page_idx
+-- (page_id) are unaffected and stay as-is.
+
+-- 6. Drop the now-dead inline column. This also drops the
+--    page_tags_tag_nonempty CHECK; the 1..=64 length bound is now
+--    enforced by `repo::tag::upsert_by_name` before a row is created.
+ALTER TABLE page_tags DROP COLUMN tag;