docs: remove free tier, update rate limits and auth for demo+pro model

- Remove free tier from rate limits, add Demo (5/hour, watermarked)
- Update auth section: remove free-tier key mention, link to docfast.dev
- Update getting started: demo → upgrade to Pro → use API key
- Add deprecated: true to /v1/signup/free swagger annotation
- Regenerate openapi.json

dist/services/db.js

@@ -146,6 +146,8 @@ export async function initDatabase() {
       );
       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,