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/client_ip.rs

@@ -0,0 +1,137 @@
+//! Resolves the originating client IP for each request and exposes it to
+//! handlers via the [`ClientIp`] request extension.
+//!
+//! Resolution order (most-trusted first):
+//! 1. `X-Forwarded-For` — first hop (the original client). Normogen is deployed
+//!    behind a reverse proxy on Solaria, so this is the primary source.
+//! 2. `X-Real-IP` — common single-IP proxy header.
+//! 3. The peer address from Axum's `ConnectInfo<SocketAddr>` (direct connection).
+//! 4. `"0.0.0.0"` only if none of the above are present.
+
+#![allow(dead_code)]
+use axum::{
+    extract::ConnectInfo, extract::Request, http::HeaderMap, middleware::Next, response::Response,
+};
+use std::net::SocketAddr;
+
+/// Header carrying the originating client chain (proxy-populated).
+const X_FORWARDED_FOR: &str = "x-forwarded-for";
+/// Header some proxies set to the single originating IP.
+const X_REAL_IP: &str = "x-real-ip";
+/// Fallback when no IP can be determined.
+const UNKNOWN_IP: &str = "0.0.0.0";
+
+/// Extractor carrying the resolved client IP, inserted by [`client_ip_middleware`].
+#[derive(Debug, Clone)]
+pub struct ClientIp(pub String);
+
+impl ClientIp {
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+}
+
+impl std::ops::Deref for ClientIp {
+    type Target = str;
+    fn deref(&self) -> &str {
+        &self.0
+    }
+}
+
+/// Resolve the client IP from request headers, falling back to the peer socket.
+///
+/// Pure and unit-testable: pass the headers and an optional `ConnectInfo`.
+pub fn extract_client_ip(headers: &HeaderMap, conn: Option<&ConnectInfo<SocketAddr>>) -> String {
+    // X-Forwarded-For: "client, proxy1, proxy2" — the leftmost entry is the
+    // original client. Only validate it looks like an IP; take the first token.
+    if let Some(xff) = headers.get(X_FORWARDED_FOR).and_then(|h| h.to_str().ok()) {
+        if let Some(first) = xff.split(',').map(str::trim).find(|s| !s.is_empty()) {
+            return first.to_string();
+        }
+    }
+
+    // X-Real-IP: single IP.
+    if let Some(ip) = headers.get(X_REAL_IP).and_then(|h| h.to_str().ok()) {
+        let ip = ip.trim();
+        if !ip.is_empty() {
+            return ip.to_string();
+        }
+    }
+
+    // Direct peer address, when ConnectInfo is wired (see main.rs).
+    if let Some(info) = conn {
+        return info.0.ip().to_string();
+    }
+
+    UNKNOWN_IP.to_string()
+}
+
+/// Middleware that computes the client IP once and stashes it in request
+/// extensions so handlers can pull it out with `Extension(ClientIp(..))`.
+pub async fn client_ip_middleware(mut req: Request, next: Next) -> Response {
+    let conn = req.extensions().get::<ConnectInfo<SocketAddr>>();
+    let ip = extract_client_ip(req.headers(), conn);
+    req.extensions_mut().insert(ClientIp(ip));
+    next.run(req).await
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use axum::extract::ConnectInfo;
+    use std::net::{IpAddr, Ipv4Addr, SocketAddr};
+
+    fn headers() -> HeaderMap {
+        HeaderMap::new()
+    }
+
+    fn conn() -> ConnectInfo<SocketAddr> {
+        ConnectInfo(SocketAddr::new(
+            IpAddr::V4(Ipv4Addr::new(203, 0, 113, 7)),
+            12345,
+        ))
+    }
+
+    #[test]
+    fn prefers_x_forwarded_for_first_hop() {
+        let mut h = headers();
+        h.insert(
+            X_FORWARDED_FOR,
+            "198.51.100.2, 10.0.0.1, 10.0.0.2".parse().unwrap(),
+        );
+        assert_eq!(extract_client_ip(&h, Some(&conn())), "198.51.100.2");
+    }
+
+    #[test]
+    fn falls_back_to_x_real_ip_when_no_xff() {
+        let mut h = headers();
+        h.insert(X_REAL_IP, "198.51.100.9".parse().unwrap());
+        assert_eq!(extract_client_ip(&h, Some(&conn())), "198.51.100.9");
+    }
+
+    #[test]
+    fn xff_takes_precedence_over_x_real_ip() {
+        let mut h = headers();
+        h.insert(X_FORWARDED_FOR, "198.51.100.2".parse().unwrap());
+        h.insert(X_REAL_IP, "10.0.0.99".parse().unwrap());
+        assert_eq!(extract_client_ip(&h, Some(&conn())), "198.51.100.2");
+    }
+
+    #[test]
+    fn uses_socket_when_no_proxy_headers() {
+        assert_eq!(extract_client_ip(&headers(), Some(&conn())), "203.0.113.7");
+    }
+
+    #[test]
+    fn unknown_when_nothing_available() {
+        assert_eq!(extract_client_ip(&headers(), None), UNKNOWN_IP);
+    }
+
+    #[test]
+    fn ignores_blank_xff_entries() {
+        let mut h = headers();
+        // Leading whitespace/empty entry should be skipped, not returned blank.
+        h.insert(X_FORWARDED_FOR, "  , 10.0.0.5".parse().unwrap());
+        assert_eq!(extract_client_ip(&h, None), "10.0.0.5");
+    }
+}