docs(arch): show Caddy + X-Forwarded-Proto in auth-flow diagram

Adds the Caddy hop to seq-auth-flow.puml and surfaces the two
production-relevant header behaviours:

  - Caddy terminates TLS and forwards X-Forwarded-Proto: https
  - Spring Boot trusts this header (server.forward-headers-strategy:
    native, ForwardedRequestCustomizer at the Jetty layer), so
    request.getScheme() returns "https"
  - The Set-Cookie response carries the Secure flag because the
    observed scheme is https — without forward-headers-strategy this
    would silently drop to plain http and the cookie would lose Secure

Closes the doc-currency gap flagged in the Markus review on PR #499:
"Auth flow change → docs/architecture/c4/seq-auth-flow.puml".

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -1,26 +1,49 @@
 @startuml
-title Authentication Flow
+title Authentication Flow (behind Caddy reverse proxy)
 
 actor User
 participant Browser
+participant "Caddy (TLS termination)" as Caddy
 participant "Frontend (SvelteKit)" as Frontend
 participant "Backend (Spring Boot)" as Backend
 participant PostgreSQL as DB
 
 User -> Browser: Enter email + password
-Browser -> Frontend: POST /login (form action)
+Browser -> Caddy: HTTPS POST /login (form action)
+note right of Caddy
+  Caddy terminates TLS and forwards
+  to Frontend over HTTP with:
+    X-Forwarded-Proto: https
+    X-Forwarded-For: <client IP>
+    X-Forwarded-Host: archiv.raddatz.cloud
+end note
+Caddy -> Frontend: HTTP POST /login\n+ X-Forwarded-Proto: https
 Frontend -> Frontend: Base64 encode "email:password"
-Frontend -> Backend: GET /api/users/me\nAuthorization: Basic <token>
+Frontend -> Backend: GET /api/users/me\nAuthorization: Basic <token>\n+ X-Forwarded-Proto: https
+note right of Backend
+  server.forward-headers-strategy: native
+  Jetty's ForwardedRequestCustomizer
+  reads X-Forwarded-Proto so
+  request.getScheme() returns "https".
+end note
 Backend -> Backend: Spring Security parses Basic Auth
 Backend -> DB: SELECT user WHERE email=?
 DB --> Backend: AppUser + groups + permissions
 Backend -> Backend: BCrypt.matches(password, hash)
 Backend --> Frontend: 200 OK — UserDTO
-Frontend -> Browser: Set-Cookie: auth_token=<base64>\n(httpOnly, SameSite=strict, maxAge=86400)
+Frontend -> Caddy: Set-Cookie: auth_token=<base64>\n(httpOnly, **Secure**, SameSite=strict, maxAge=86400)
-Browser -> Frontend: GET / (next request)
+note right of Frontend
+  Secure flag is set because the
+  request scheme observed by the
+  app is https (forwarded by Caddy).
+end note
+Caddy -> Browser: HTTPS 200 + Set-Cookie
+Browser -> Caddy: HTTPS GET / (next request)
+Caddy -> Frontend: HTTP GET / + X-Forwarded-Proto: https
 Frontend -> Frontend: hooks.server.ts reads auth_token cookie
 Frontend -> Backend: GET /api/users/me\nAuthorization: Basic <token>
 Backend --> Frontend: 200 OK — user in event.locals
-Frontend --> Browser: Render page with user context
+Frontend --> Caddy: rendered page
+Caddy --> Browser: HTTPS 200
 
 @enduml