fix: webhook idempotency — unique index on stripe_customer_id + UPSERT + DB dedup on success page

- Add partial unique index on api_keys(stripe_customer_id) WHERE NOT NULL
- Use INSERT ... ON CONFLICT in createProKey for cross-pod dedup
- Add findKeyByCustomerId() to query DB directly
- Success page checks DB before creating key (survives pod restarts)
- Refresh in-memory cache after UPSERT

src/services/db.ts

@@ -162,6 +162,8 @@ export async function initDatabase(): Promise<void> {
       );
       CREATE INDEX IF NOT EXISTS idx_api_keys_email ON api_keys(email);
       CREATE INDEX IF NOT EXISTS idx_api_keys_stripe ON api_keys(stripe_customer_id);
+      CREATE UNIQUE INDEX IF NOT EXISTS idx_api_keys_stripe_unique
+        ON api_keys(stripe_customer_id) WHERE stripe_customer_id IS NOT NULL;
 
       CREATE TABLE IF NOT EXISTS verifications (
         id SERIAL PRIMARY KEY,