Add auth module with register, login, and JWT (#2)

## Summary

- Auth endpoints: POST /api/v1/auth/register, POST /api/v1/auth/login,
GET /api/v1/auth/me
- Reorganized project structure to match architecture doc (api/v1/,
db/entities/, db/migrations/)
- Typed AppConfig with `#[derive(Config)]` for HOST, PORT, DATABASE_URL
- Migration converting all PKs to serial id + UUID pid for
internal/external separation
- Manual SeaORM entity for users (schema! macro doesn't support UUID
fields yet)
- 8 integration tests using Rapina TestClient against real Postgres
- OpenAPI spec exported and committed
- CI: added rapina openapi check and rapina doctor jobs

## Test plan

- [x] `cargo test` passes all 8 auth integration tests locally
- [x] `cargo clippy -- -D warnings` clean
- [x] `cargo fmt --all -- --check` clean
- [x] `rapina doctor` passes (1 warning on /health errors, acceptable)
- [x] `rapina openapi check` passes
- [ ] CI pipeline passes on GitHub

---------

Signed-off-by: arferreira <arfs.antonio@gmail.com>

Author: arferreira

.env.example

@@ -1,3 +1,5 @@
 DATABASE_URL=postgres://reeverb:secret@localhost:5432/reeverb
 JWT_SECRET=change-me-in-production
+HOST=0.0.0.0
+PORT=3000
 RUST_LOG=info

Cargo.lock

@@ -18,6 +18,7 @@ serde_json = "1"
 uuid = { version = "1", features = ["v4", "serde"] }
 chrono = { version = "0.4", features = ["serde"] }
 dotenvy = "0.15"
+bcrypt = "0.16"
 
 [profile.release]
 lto = true