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 ✅

backend/internal/sync/hlc_test.go

@@ -0,0 +1,61 @@
+package sync
+
+import (
+	"testing"
+)
+
+func TestTick_StrictlyMonotonic(t *testing.T) {
+	// Reset global state.
+	clock.mu.Lock()
+	clock.last = 0
+	clock.mu.Unlock()
+
+	prev := int64(0)
+	for i := 0; i < 1000; i++ {
+		v := Tick(0)
+		if v <= prev {
+			t.Fatalf("iteration %d: Tick() = %d, want > %d", i, v, prev)
+		}
+		prev = v
+	}
+}
+
+func TestTick_CausalOrdering(t *testing.T) {
+	clock.mu.Lock()
+	clock.last = 0
+	clock.mu.Unlock()
+
+	// Simulate a client HLC far in the future.
+	future := int64(9_999_999_999) << counterBits
+	v := Tick(future)
+	if !After(v, future) {
+		t.Fatalf("Tick(future) = %d, want > %d", v, future)
+	}
+}
+
+func TestTick_NoDuplicates(t *testing.T) {
+	clock.mu.Lock()
+	clock.last = 0
+	clock.mu.Unlock()
+
+	seen := make(map[int64]bool)
+	for i := 0; i < 10_000; i++ {
+		v := Tick(0)
+		if seen[v] {
+			t.Fatalf("duplicate HLC value %d at iteration %d", v, i)
+		}
+		seen[v] = true
+	}
+}
+
+func TestAfter(t *testing.T) {
+	if !After(2, 1) {
+		t.Fatal("After(2,1) must be true")
+	}
+	if After(1, 2) {
+		t.Fatal("After(1,2) must be false")
+	}
+	if After(1, 1) {
+		t.Fatal("After(1,1) must be false")
+	}
+}