Backend Phase C: Sync-Kern + Caddy behind-proxy

Sync-Kern:
- internal/sync/hlc.go: Hybrid Logical Clock (wall_ms<<16|counter)
  Tick/Now/After, global mutex, strikt monoton + kausal korrekt
- internal/sync/hlc_test.go: Unit-Tests (monoton, kausal, keine Duplikate)

Store-Schicht:
- internal/store/opstore.go: AppendOps idempotent via UNIQUE(client_id,
  client_seq) ON CONFLICT DO NOTHING; LWW-Projektion (list_create/
  rename/delete, item_add/update/remove) in derselben Transaktion;
  PullOps mit Cursor (seq > since, 500er Pages)
- internal/store/liststore.go: CreateList / GetLists / GetList
- internal/store/itemstore.go: GetItems (nicht-gelöschte Items)
- internal/store/suggeststore.go: Search (pg_trgm + LIKE-fallback, 10)

HTTP-Handler:
- internal/httpapi/lists.go: GET/POST /api/lists, GET /api/lists/{id}
- internal/httpapi/ops.go: POST /api/lists/{id}/ops (Push),
  GET /api/lists/{id}/ops (Pull ?since=)
- internal/httpapi/suggest.go: GET /api/suggestions?q=
- internal/httpapi/api.go: alle Routen verdrahtet (RequireAuth)

Deployment:
- deploy/Caddyfile.behind-proxy: auto_https off, trusted_proxies
- deploy/Caddyfile: X-Forwarded-Proto hinzugefügt, Kommentar aktualisiert
- deploy/docker-compose.yml: CADDY_HTTP_PORT + CADDY_HTTPS_PORT
- deploy/.env.example: Caddy-Port-Variablen dokumentiert

go build ./... && go vet ./... && go test ./... ✅
HLC-Tests: monoton, kausal, keine Duplikate ✅
AGENTS.md: Phase C vollständig ✅

deploy/docker-compose.yml

@@ -4,7 +4,7 @@
 #   docker compose up -d --build
 #
 # Services:
-#   caddy    public reverse proxy with automatic HTTPS (Let's Encrypt)
+#   caddy    reverse proxy (auto-HTTPS by default; see "Behind a Reverse Proxy" below)
 #   backend  mitbringsl Go API (image builds server + migrate binaries)
 #   migrate  one-shot migration runner, must finish before backend starts
 #   db       PostgreSQL 16
@@ -81,10 +81,22 @@ services:
   caddy:
     image: caddy:2-alpine
     restart: unless-stopped
+    # --- Standalone mode (default): Caddy handles TLS via Let's Encrypt ----
+    # Exposes ports 80 (ACME HTTP-01 challenge) and 443 (HTTPS + HTTP/3).
     ports:
-      - "80:80"        # ACME HTTP-01 challenge + redirect
-      - "443:443"
-      - "443:443/udp"  # HTTP/3
+      - "${CADDY_HTTP_PORT:-80}:80"
+      - "${CADDY_HTTPS_PORT:-443}:443"
+      - "${CADDY_HTTPS_PORT:-443}:443/udp"   # HTTP/3
+    # --- Behind a Reverse Proxy mode ----------------------------------------
+    # If your own proxy handles TLS, set CADDY_HTTP_PORT to the port your proxy
+    # forwards to (e.g. 8880), and mount Caddyfile.behind-proxy instead:
+    #
+    #   CADDY_HTTP_PORT=8880
+    #   volumes:
+    #     - ./Caddyfile.behind-proxy:/etc/caddy/Caddyfile:ro
+    #
+    # Also remove the CADDY_HTTPS_PORT ports above and comment out this note.
+    # -------------------------------------------------------------------------
     volumes:
       - ./Caddyfile:/etc/caddy/Caddyfile:ro
       - caddy_data:/data