feat: Stripe integration complete - products created, DB migration, payment_failed handler, credit buttons wired

- Create Stripe products/prices (Starter/Pro/Business monthly+yearly)
- Fix CRITICAL bug: add subscription_ends_at + cancel_at_period_end columns to users table
- Alembic migration: f6a7b8c9d0e1_add_subscription_ends_at_cancel_at_period_end
- Fix: implement handle_payment_failed() to set subscription_status=PAST_DUE
- Fix: harmonize .env.production Stripe variable names to match pricing_config.py
- Fix: add missing FRONTEND_URL and STRIPE_PUBLISHABLE_KEY to .env.production
- Add all Stripe Price IDs (test mode) to .env.production
- Wire credit purchase buttons to /api/v1/auth/create-credits-checkout
- Dashboard sync post-checkout was already implemented (no change needed)

Stripe test keys: configured in .env.production
Webhook: must be configured on server via stripe CLI or Stripe Dashboard
Webhook URL: https://wordly.art/api/v1/auth/webhook/stripe

alembic/versions/f6a7b8c9d0e1_add_subscription_ends_at_cancel_at_period_end.py

@@ -0,0 +1,37 @@
+"""Add subscription_ends_at and cancel_at_period_end to users
+
+Revision ID: f6a7b8c9d0e1
+Revises: e5b2c9d1f4a8
+Create Date: 2026-05-31
+
+Fixes critical bug: these columns were used in update_user() but never
+persisted to the database, causing subscription end dates and cancellation
+state to be lost on server restart.
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers
+revision = "f6a7b8c9d0e1"
+down_revision = "e5b2c9d1f4a8"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "users",
+        sa.Column("subscription_ends_at", sa.DateTime(), nullable=True),
+    )
+    op.add_column(
+        "users",
+        sa.Column("cancel_at_period_end", sa.Boolean(), nullable=False, server_default="false"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("users", "cancel_at_period_end")
+    op.drop_column("users", "subscription_ends_at")