docs(arch): update auth sequence diagram to Phase 2 (CSRF, rate limit, revocation)

Extends the diagram from ADR-020 Phase 1 to cover:
- Rate limiter gate before credential validation in login
- CSRF double-submit cookie handshake for mutating requests
- Session revocation on password change (revokeOtherSessions) and
  password reset (revokeAllSessions)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/architecture/c4/seq-auth-flow.puml

@@ -1,9 +1,9 @@
 @startuml
 title Authentication Flow (Spring Session JDBC, behind Caddy reverse proxy)
 note over Browser, DB
-  Phase 1 of the auth rewrite (ADR-020 / #523).
+  Phase 2 of the auth rewrite (ADR-020, ADR-022 / #523, #524).
-  Replaces the Basic-credentials-in-cookie model
+  Adds CSRF double-submit cookies, login rate limiting, and
-  with an opaque server-side session id (fa_session).
+  session revocation on password change/reset.
 end note
 
 actor User
@@ -11,9 +11,10 @@ participant Browser
 participant "Caddy (TLS termination)" as Caddy
 participant "Frontend (SvelteKit)" as Frontend
 participant "Backend (Spring Boot)" as Backend
+participant "LoginRateLimiter\n(Caffeine+Bucket4j)" as RateLimiter
 participant "spring_session\n(PostgreSQL)" as DB
 
-== Login ==
+== Login (with rate limiting + CSRF bootstrap) ==
 User -> Browser: Enter email + password
 Browser -> Caddy: HTTPS POST /?/login (form action)
 note right of Caddy
@@ -30,19 +31,46 @@ note right of Backend
   → request.getScheme() = "https"
   → Secure cookie flag set automatically.
 end note
-Backend -> Backend: AuthenticationManager\nauthenticate(email, password)
+Backend -> RateLimiter: checkAndConsume(ip, email)\n[10/15min per ip+email; 20/15min per ip]
-Backend -> DB: SELECT user WHERE email=?
+alt Rate limit exceeded
-DB --> Backend: AppUser + groups + permissions
+  RateLimiter --> Backend: throw DomainException(TOO_MANY_LOGIN_ATTEMPTS)
-Backend -> Backend: BCrypt.matches(password, hash)\n(timing-safe: dummy hash on miss)
+  Backend -> Backend: AuditService.log(LOGIN_RATE_LIMITED, {ip, email})
-Backend -> Backend: getSession(true).setAttribute(\n  SPRING_SECURITY_CONTEXT, ctx)
+  Backend --> Frontend: 429 Too Many Requests\n{"code":"TOO_MANY_LOGIN_ATTEMPTS"}
-Backend -> DB: INSERT spring_session\n+ spring_session_attributes
+  Frontend --> Browser: Show rate-limit error
-Backend -> Backend: AuditService.log(LOGIN_SUCCESS,\n  {userId, ip, ua})
+else Under limit
-Backend --> Frontend: 200 OK — AppUser\nSet-Cookie: fa_session=<opaque>;\n  Path=/; HttpOnly; SameSite=Strict; Secure
+  Backend -> Backend: AuthenticationManager\nauthenticate(email, password)
-Frontend -> Frontend: Parse Set-Cookie, re-emit fa_session\n(matches backend attrs)
+  Backend -> DB: SELECT user WHERE email=?
-Frontend --> Caddy: 303 → /\nSet-Cookie: fa_session=<opaque>
+  DB --> Backend: AppUser + groups + permissions
-Caddy --> Browser: HTTPS 303 + Set-Cookie
+  Backend -> Backend: BCrypt.matches(password, hash)\n(timing-safe: dummy hash on miss)
+  Backend -> Backend: getSession(true).setAttribute(\n  SPRING_SECURITY_CONTEXT, ctx)
+  Backend -> DB: INSERT spring_session\n+ spring_session_attributes
+  Backend -> RateLimiter: invalidateOnSuccess(ip, email)
+  Backend -> Backend: AuditService.log(LOGIN_SUCCESS,\n  {userId, ip, ua})
+  Backend --> Frontend: 200 OK — AppUser\nSet-Cookie: fa_session=<opaque>;\n  Path=/; HttpOnly; SameSite=Strict; Secure\nSet-Cookie: XSRF-TOKEN=<token>;\n  Path=/; SameSite=Strict; Secure
+  Frontend -> Frontend: Parse Set-Cookie, re-emit fa_session\n(matches backend attrs)
+  Frontend --> Caddy: 303 → /\nSet-Cookie: fa_session=<opaque>
+  Caddy --> Browser: HTTPS 303 + Set-Cookie
+end
 
-== Authenticated request ==
+== Authenticated mutating request (CSRF double-submit) ==
+note over Browser, Backend
+  handleFetch in hooks.client.ts reads the XSRF-TOKEN cookie
+  and injects X-XSRF-TOKEN header on every POST/PUT/PATCH/DELETE.
+end note
+Browser -> Caddy: HTTPS POST /api/...\nCookie: fa_session=<opaque>; XSRF-TOKEN=<token>\nX-XSRF-TOKEN: <token>
+Caddy -> Backend: HTTP POST /api/...\n+ Cookie + X-XSRF-TOKEN
+alt X-XSRF-TOKEN missing or mismatched
+  Backend --> Caddy: 403 Forbidden\n{"code":"CSRF_TOKEN_MISSING"}
+  Caddy --> Browser: HTTPS 403
+else CSRF valid
+  Backend -> DB: SELECT * FROM spring_session WHERE SESSION_ID = ?
+  DB --> Backend: session row
+  Backend -> Backend: Process request
+  Backend --> Caddy: 2xx response + refreshed XSRF-TOKEN cookie
+  Caddy --> Browser: HTTPS 2xx
+end
+
+== Authenticated read request ==
 Browser -> Caddy: HTTPS GET /\nCookie: fa_session=<opaque>
 Caddy -> Frontend: HTTP GET / + Cookie + X-Forwarded-Proto: https
 Frontend -> Frontend: hooks.server.ts reads fa_session
@@ -61,6 +89,28 @@ else Session expired (idle > 8h) or unknown
   Caddy --> Browser: HTTPS 302
 end
 
+== Password change (revoke other sessions) ==
+Browser -> Backend: POST /api/users/me/password\n{currentPassword, newPassword}\n+ X-XSRF-TOKEN
+Backend -> Backend: Verify currentPassword
+Backend -> DB: UPDATE app_users SET password_hash = ?
+Backend -> DB: DELETE spring_session WHERE principal = ?\n  AND session_id != <current>
+note right of Backend
+  revokeOtherSessions: caller stays logged in,
+  all other devices are signed out.
+end note
+Backend --> Browser: 204 No Content
+
+== Password reset (revoke all sessions) ==
+Browser -> Backend: POST /api/auth/reset-password\n{token, newPassword}
+Backend -> Backend: Verify reset token
+Backend -> DB: UPDATE app_users SET password_hash = ?
+Backend -> DB: DELETE spring_session WHERE principal = ?
+note right of Backend
+  revokeAllSessions: unauthenticated caller has
+  no session to preserve — all sessions wiped.
+end note
+Backend --> Browser: 204 No Content
+
 == Logout ==
 Browser -> Caddy: HTTPS POST /logout
 Caddy -> Frontend: HTTP POST /logout\nCookie: fa_session=<opaque>