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/middleware/auth.rs

@@ -8,6 +8,7 @@ use axum::{
     middleware::Next,
     response::Response,
 };
+use mongodb::bson::oid::ObjectId;
 
 pub async fn jwt_auth_middleware(
     State(state): State<AppState>,
@@ -29,12 +30,26 @@ pub async fn jwt_auth_middleware(
 
     let token = &auth_header[7..]; // Remove "Bearer " prefix
 
-    // Verify token
+    // Verify signature and expiry.
     let claims = state
         .jwt_service
         .validate_token(token)
         .map_err(|_| StatusCode::UNAUTHORIZED)?;
 
+    // Reject tokens whose version is stale (e.g. issued before a password
+    // change). The user's current version is cached briefly so we don't hit
+    // Mongo on every request; a missing user (deleted) also means 401.
+    let user_oid = ObjectId::parse_str(&claims.sub).map_err(|_| StatusCode::UNAUTHORIZED)?;
+    let current_version = state
+        .token_version_cache
+        .get_or_load(&user_oid, &state.db)
+        .await
+        .map_err(|_| StatusCode::UNAUTHORIZED)?;
+    match current_version {
+        Some(v) if v == claims.token_version => {}
+        _ => return Err(StatusCode::UNAUTHORIZED),
+    }
+
     // Add claims to request extensions for handlers to use
     req.extensions_mut().insert(claims);