feat: track imported accounts per upstream key across platforms

Add upstream_key_account_links table mapping generated keys to remote
accounts per website/platform, surface imported_accounts on key
responses, and update website sync/routers to manage the links.

backend/app/database.py

@@ -47,6 +47,8 @@ def init_db():
     _migrate_custom_pages()
     _migrate_upstreams()
     _migrate_upstream_generated_keys()
+    _migrate_website_group_bindings_platform()
+    _migrate_upstream_key_account_links()
 
 
 # ── 已有数据库幂等索引迁移 ─────────────────────────────────
@@ -197,3 +199,43 @@ def _migrate_upstream_generated_keys():
     except Exception:
         logger = __import__("logging").getLogger(__name__)
         logger.warning("could not create unique indexes on upstream_generated_keys (non-fatal)")
+
+
+def _migrate_upstream_key_account_links():
+    """Create the multi-platform account mapping table and indexes."""
+    inspector = inspect(engine)
+    if "upstream_key_account_links" not in inspector.get_table_names():
+        with engine.begin() as conn:
+            conn.execute(text("""
+                CREATE TABLE IF NOT EXISTS upstream_key_account_links (
+                    id INTEGER PRIMARY KEY AUTOINCREMENT,
+                    upstream_key_id INTEGER NOT NULL REFERENCES upstream_generated_keys(id) ON DELETE CASCADE,
+                    website_id INTEGER NOT NULL REFERENCES websites(id) ON DELETE CASCADE,
+                    target_group VARCHAR(255) NOT NULL DEFAULT 'default',
+                    platform VARCHAR(64) NOT NULL,
+                    remote_account_id VARCHAR(255) NOT NULL,
+                    status VARCHAR(32) NOT NULL DEFAULT 'active',
+                    created_at DATETIME,
+                    updated_at DATETIME
+                )
+            """))
+
+    with engine.begin() as conn:
+        conn.execute(text("""
+            CREATE UNIQUE INDEX IF NOT EXISTS uq_key_website_platform
+            ON upstream_key_account_links(upstream_key_id, website_id, platform)
+        """))
+        conn.execute(text("""
+            CREATE UNIQUE INDEX IF NOT EXISTS uq_website_remote_account
+            ON upstream_key_account_links(website_id, remote_account_id)
+        """))
+
+def _migrate_website_group_bindings_platform():
+    """Add platform column to website_group_bindings if missing."""
+    inspector = inspect(engine)
+    if "website_group_bindings" not in inspector.get_table_names():
+        return
+    columns = {col["name"] for col in inspector.get_columns("website_group_bindings")}
+    if "platform" not in columns:
+        with engine.begin() as conn:
+            conn.execute(text("ALTER TABLE website_group_bindings ADD COLUMN platform VARCHAR(64)"))