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,

src/services/keys.ts

@@ -86,6 +86,7 @@ export async function createFreeKey(email?: string): Promise<ApiKey> {
 }
 
 export async function createProKey(email: string, stripeCustomerId: string): Promise<ApiKey> {
+  // Check in-memory cache first (fast path)
   const existing = keysCache.find((k) => k.stripeCustomerId === stripeCustomerId);
   if (existing) {
     existing.tier = "pro";
@@ -93,19 +94,36 @@ export async function createProKey(email: string, stripeCustomerId: string): Pro
     return existing;
   }
 
+  // UPSERT: handles duplicate webhooks across pods via DB unique index
+  const newKey = generateKey("df_pro");
+  const now = new Date().toISOString();
+
+  const result = await queryWithRetry(
+    `INSERT INTO api_keys (key, tier, email, created_at, stripe_customer_id)
+     VALUES ($1, $2, $3, $4, $5)
+     ON CONFLICT (stripe_customer_id) WHERE stripe_customer_id IS NOT NULL
+     DO UPDATE SET tier = 'pro'
+     RETURNING key, tier, email, created_at, stripe_customer_id`,
+    [newKey, "pro", email, now, stripeCustomerId]
+  );
+
+  const row = result.rows[0];
   const entry: ApiKey = {
-    key: generateKey("df_pro"),
-    tier: "pro",
-    email,
-    createdAt: new Date().toISOString(),
-    stripeCustomerId,
+    key: row.key,
+    tier: row.tier as "free" | "pro",
+    email: row.email,
+    createdAt: row.created_at instanceof Date ? row.created_at.toISOString() : row.created_at,
+    stripeCustomerId: row.stripe_customer_id || undefined,
   };
 
-  await queryWithRetry(
-    "INSERT INTO api_keys (key, tier, email, created_at, stripe_customer_id) VALUES ($1, $2, $3, $4, $5)",
-    [entry.key, entry.tier, entry.email, entry.createdAt, entry.stripeCustomerId]
-  );
-  keysCache.push(entry);
+  // Refresh in-memory cache
+  const cacheIdx = keysCache.findIndex((k) => k.stripeCustomerId === stripeCustomerId);
+  if (cacheIdx >= 0) {
+    keysCache[cacheIdx] = entry;
+  } else {
+    keysCache.push(entry);
+  }
+
   return entry;
 }
 
@@ -119,6 +137,23 @@ export async function downgradeByCustomer(stripeCustomerId: string): Promise<boo
   return false;
 }
 
+export async function findKeyByCustomerId(stripeCustomerId: string): Promise<ApiKey | null> {
+  // Check DB directly — survives pod restarts unlike in-memory cache
+  const result = await queryWithRetry(
+    "SELECT key, tier, email, created_at, stripe_customer_id FROM api_keys WHERE stripe_customer_id = $1 LIMIT 1",
+    [stripeCustomerId]
+  );
+  if (result.rows.length === 0) return null;
+  const r = result.rows[0];
+  return {
+    key: r.key,
+    tier: r.tier as "free" | "pro",
+    email: r.email,
+    createdAt: r.created_at instanceof Date ? r.created_at.toISOString() : r.created_at,
+    stripeCustomerId: r.stripe_customer_id || undefined,
+  };
+}
+
 export function getAllKeys(): ApiKey[] {
   return [...keysCache];
 }