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/Caddyfile

@@ -1,25 +1,27 @@
-# Caddyfile for mitbringsl.
+# Caddyfile for mitbringsl – standalone mode (default).
 # Caddy automatically obtains and renews a Let's Encrypt certificate when the
 # site address is a real domain. For local development it falls back to an
 # internal CA / self-signed cert automatically.
+#
+# To run behind your own reverse proxy instead, mount Caddyfile.behind-proxy.
 {
-	# email you@example.com   # optional, for ACME account
+	# email you@example.com   # optional, for ACME account registration
 }
 
 {$SITE_ADDRESS:localhost} {
 	reverse_proxy backend:8080 {
-		header_up X-Real-IP {remote_host}
-		header_up X-Forwarded-For {remote_host}
+		header_up X-Real-IP        {remote_host}
+		header_up X-Forwarded-For  {remote_host}
+		header_up X-Forwarded-Proto {scheme}
 	}
 
-	# Useful default headers
+	# Useful default security headers
 	header {
 		Strict-Transport-Security "max-age=31536000"
 		X-Content-Type-Options "nosniff"
 		Referrer-Policy "no-referrer"
 	}
 
-	# Health endpoint passthrough already handled by backend; keep it simple.
 	request_body {
 		max_size 2MB
 	}