fix(backend): P0 security hardening pass

Address the four P0 security items from the project review:

* token_version validation (#1): the JWT middleware now rejects access tokens
  whose token_version claim is stale (e.g. issued before a password change).
  A short-TTL (30s) in-memory cache (TokenVersionCache) avoids a Mongo lookup
  per request; credential changes invalidate the cache immediately on the
  handling instance.

* Fail-fast config (#2): add APP_ENVIRONMENT (development|production). In
  production the server refuses to boot unless JWT_SECRET and ENCRYPTION_KEY
  are set to non-default values; development keeps the insecure defaults with
  a warning.

* Real client IP in audit logs (#4): new client_ip middleware resolves the
  originating IP (X-Forwarded-For > X-Real-IP > ConnectInfo socket) and
  exposes it via a ClientIp extractor. All five hardcoded "0.0.0.0" audit
  calls are replaced, and the missing PasswordChanged audit event is added to
  change_password. axum::serve now uses into_make_service_with_connect_info.

* Refresh token persistence (#5): refresh tokens are now stored hashed in
  MongoDB (RefreshTokenRepository) instead of an in-memory map lost on restart.
  Added /api/auth/refresh (with rotation + token_version check) and
  /api/auth/logout routes; register/login return a refresh_token; password
  change/recovery revoke all of a user's refresh tokens. JwtService now honors
  JwtConfig expiries instead of hardcoding 15min/30d, and the dead in-memory
  refresh store is removed.

Also: wire up DatabaseInitializer (was never called), fix the refresh_tokens
index to tokenHash + add an expiresAt TTL index, add sha2 dep.

Rate limiting (#3) is deferred per scope; the stub remains.

Verified: cargo fmt --check clean, cargo build/clippy --all-targets clean,
18 unit tests pass (9 new). Integration tests (tests/*) still need a live
server — fixing them is tracked as P1.

BREAKING CHANGE: AuthResponse now includes a refresh_token field.

backend/src/handlers/mod.rs

@@ -9,7 +9,7 @@ pub mod shares;
 pub mod users;
 
 // Re-export commonly used handler functions
-pub use auth::{login, recover_password, register};
+pub use auth::{login, logout, recover_password, refresh, register};
 pub use health::{health_check, ready_check};
 pub use health_stats::{
     create_health_stat, delete_health_stat, get_health_stat, get_health_trends, list_health_stats,