Refactor API contracts to Zod, zod-to-openapi, and Scalar.

Replace hand-written OpenAPI and Swagger UI with schema-driven docs at /api-docs and Zod validation on request bodies.

Co-authored-by: Cursor <cursoragent@cursor.com>

.env.example

@@ -23,5 +23,7 @@ DEMO_MODE=false
 # Feed recorder (NDJSON to data/)
 RECORD_FEED=false
 
+# API docs — Scalar at GET /api-docs (OpenAPI generated from Zod; no env vars required)
+
 # Sentry — https://sentry.io (project: arbpulse)
 SENTRY_DSN=