feat: REQ-002/REQ-010 implementation — assets.tag_ids column + GET /v1/assets admin catalog (BAT-012)

- REQ-002 scope redefinition: portfolios.tag_ids → assets.tag_ids UUID[] column with GIN index
- REQ-010 augmentation: GET /v1/assets redesigned as admin catalog endpoint (admin-only, no token_type branching)
- Asset tagging infrastructure: tag_ids column, archived flag synthesis for inactive tags, list_assets N+1 optimization
- DB migration 20260521_0023: assets.tag_ids UUID[] NOT NULL DEFAULT '{}', idx_assets_tag_ids_gin
- Test suite: 17 new test cases for assets.tag_ids + GET /v1/assets AC-9~12 (362 tests total)
- Documentation: runtime flows, release baseline, batch verification all updated
- BAT-012 added to REL-001 Included Batch and marked release-candidate

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

app/repositories/postgres.py

@@ -87,7 +87,8 @@ _SELECT_ASSET = """
         display_name,
         status,
         created_at,
-        updated_at
+        updated_at,
+        COALESCE(tag_ids, '{}'::uuid[])
     FROM assets
 """
 
@@ -463,6 +464,7 @@ class PostgresPortfolioRepository:
             status=row[4] or "active",
             created_at=row[5],
             updated_at=row[6],
+            tag_ids=[str(t) for t in row[7]] if len(row) > 7 and row[7] is not None else [],
         )
 
     @staticmethod
@@ -1503,7 +1505,8 @@ class PostgresPortfolioRepository:
                 """
                 INSERT INTO assets (symbol, markets, display_name, status)
                 VALUES (%s, %s, %s, %s)
-                RETURNING asset_id, symbol, markets, display_name, status, created_at, updated_at
+                RETURNING asset_id, symbol, markets, display_name, status, created_at, updated_at,
+                          COALESCE(tag_ids, '{}'::uuid[])
                 """,
                 (symbol, markets, display_name, status),
             )