feat(api): add listing conversations and text messaging

[SKonteye]

Closes #4

Members can now start a private conversation about an ACTIVE listing and exchange text messages through authenticated REST endpoints. This is the API foundation; web sign-in, messaging UI, attachments, and Socket.IO delivery remain follow-up work.

Behavior

• Add Conversation, ConversationParticipant (OWNER / INTERESTED), and Message models and a migration. A unique listing/member pair makes concurrent contact requests reuse one conversation.
• Add endpoints to start/reuse a conversation, list conversations, retrieve a conversation, and list/send messages. Lists use offset pagination with an explicit ID tie-breaker.
• Check listing eligibility under a database row lock in the creation transaction, preventing concurrent status changes from bypassing the ACTIVE requirement. Existing participants can continue after archival.
• Return 400 for self-contact, 404 for missing or rejected listings, and 409 for other non-ACTIVE listings. Conversation access by non-participants returns 404.
• Trim and validate message bodies (1–2000 characters). Persist the message and advance activity atomically; delayed sends cannot move the activity timestamp backwards.
• Reuse the safe public member shape. Conversation responses omit phone numbers, location data, and listing moderation status.
• Publish concrete OpenAPI response schemas and regenerate the shared client. Conversation client return types match the shared mutator's direct response-body behavior. Generation cleans stale models and exports.

Validation

• Passed repository lint and typecheck.
• Passed 26 unit tests and 23 end-to-end tests against local Docker PostgreSQL and Redis, including concurrent conversation reuse, participant isolation, privacy, response schemas, and monotonic activity.
• API build and API/client generation passed.
• Full repository build was attempted; web/admin Turbopack builds are blocked locally by a port-binding permission error. CI must verify the full build for this revision.

The migration preserves the hand-written PostGIS and search indexes.