feat(deployments): list deployments

[ctxswitch]

Change Summary

• Add GET /deployments for current-state deployment listing with exact service, environment, and status filters.
• Add offset pagination with limit, offset, and nullable next_offset, returning full deployment records ordered by created_at DESC, id DESC.
• Add shared request/response types, domain validation/defaulting, app-level validation before persistence, SQLite query support, HTTP query parsing, and API docs.

Risks

• next_offset is optimistic and set when a full page is returned; it does not issue an extra count query to prove another page exists.
• Rollout: existing SQLite databases get a new deployments_created_at_id_idx index during schema initialization.
• Rollback: reverting this PR removes the endpoint and index creation; the existing index can remain harmlessly in SQLite.
• Validation: make test and make lint pass.

Issues

• Closes feat(deployment): list deployments with filters and pagination #4

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