feat: add account-based subscription model with migrations

Transition from team-based to account-based multi-tenancy (Free/Pro/Team).
Migrations 016-020 create accounts, subscriptions, plan_limits, and
account_invites tables, then migrate existing users and content FKs.

New models: Account, Subscription, PlanLimits, AccountInvite.
Updated models add account_id alongside existing team_id (coexistence
for safe two-PR deployment). Permissions and deps refactored for
account_role instead of is_team_admin.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/core/config.py

@@ -52,6 +52,16 @@ class Settings(BaseSettings):
     # Registration
     REQUIRE_INVITE_CODE: bool = True  # Set to False to allow open registration
 
+    # Stripe
+    STRIPE_SECRET_KEY: Optional[str] = None
+    STRIPE_PUBLISHABLE_KEY: Optional[str] = None
+    STRIPE_WEBHOOK_SECRET: Optional[str] = None
+
+    @property
+    def stripe_enabled(self) -> bool:
+        """Check if Stripe is configured."""
+        return self.STRIPE_SECRET_KEY is not None and self.STRIPE_WEBHOOK_SECRET is not None
+
     # CORS - set FRONTEND_URL in production (e.g., https://patherly.up.railway.app)
     CORS_ORIGINS: list[str] = ["http://localhost:3000", "http://localhost:5173", "http://localhost:5174"]
     FRONTEND_URL: Optional[str] = None