feat(api): fall back to next port (3000→3010) when the port is in use

The API server bound a fixed port (3000) and died with EADDRINUSE when it
was taken — painful when running multiple dispatch instances (e.g. testing
several feature branches at once). Replace the static default export with an
explicit Bun.serve retry loop that increments the port by one on EADDRINUSE,
from START_PORT (PORT env or 3000) up to MAX_PORT (3010), logging the chosen
port and a hint to repoint the frontend's API URL on a bump.

Guarded by import.meta.main so importing the module (for `app`) never binds a
port. Frontend unchanged — set its API URL manually when a bump occurs.

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