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.behind-proxy

@@ -0,0 +1,41 @@
+# Caddyfile for mitbringsl – behind external reverse proxy mode.
+#
+# Use this when your own reverse proxy (nginx, Traefik, etc.) handles TLS
+# termination and routes traffic to Caddy over plain HTTP on the internal
+# Docker network.
+#
+# In docker-compose.yml, mount this file instead of the default Caddyfile:
+#   volumes:
+#     - ./Caddyfile.behind-proxy:/etc/caddy/Caddyfile:ro
+#
+# Your external proxy should set:
+#   X-Forwarded-For   <client-ip>
+#   X-Forwarded-Proto https
+#   Host              <your-domain>
+{
+	# Disable auto-HTTPS – TLS is handled by the outer proxy.
+	auto_https off
+}
+
+:80 {
+	# Trust the private-network upstream so X-Forwarded-* headers are accepted.
+	# Change to a specific IP/CIDR if your proxy has a fixed address.
+	trusted_proxies private_ranges
+
+	reverse_proxy backend:8080 {
+		# Forward the real client IP to the backend.
+		header_up X-Real-IP        {http.request.header.X-Forwarded-For}
+		header_up X-Forwarded-For  {http.request.header.X-Forwarded-For}
+		header_up X-Forwarded-Proto {http.request.header.X-Forwarded-Proto}
+	}
+
+	# Enforce a sensible request body size limit.
+	request_body {
+		max_size 2MB
+	}
+
+	log {
+		output stdout
+		format console
+	}
+}