feat(finance): Phase 4 — Tags

- tags table: name, color; transaction_tags junction table
- GET/POST /api/tags; DELETE /api/tags/:id
- POST/DELETE /api/transactions/:id/tags for per-transaction tagging
- Bulk tag/untag via /api/transactions/bulk (action: tag/untag)
- Tags returned inline with transaction list via LATERAL join
- Tag filter on Transactions page
- Bulk "Tag as..." in bulk action bar
- Tag pills + "+" picker on each transaction row
- /tags page: create with color picker, list with counts, delete

prisma/migrations/0004_tags/migration.sql

@@ -0,0 +1,13 @@
+CREATE TABLE IF NOT EXISTS tags (
+  id SERIAL PRIMARY KEY,
+  name TEXT NOT NULL UNIQUE,
+  color TEXT NOT NULL DEFAULT '#6366f1',
+  created_at TIMESTAMPTZ DEFAULT NOW()
+);
+
+CREATE TABLE IF NOT EXISTS transaction_tags (
+  transaction_id INTEGER NOT NULL REFERENCES transactions(id) ON DELETE CASCADE,
+  tag_id INTEGER NOT NULL REFERENCES tags(id) ON DELETE CASCADE,
+  created_at TIMESTAMPTZ DEFAULT NOW(),
+  PRIMARY KEY (transaction_id, tag_id)
+);