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/store/suggeststore.go

@@ -0,0 +1,46 @@
+package store
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+)
+
+const suggestLimit = 10
+
+// SuggestStore provides autocomplete search over aggregated item names.
+type SuggestStore struct {
+	pool *pgxpool.Pool
+}
+
+// NewSuggestStore creates a SuggestStore backed by the given pool.
+func NewSuggestStore(pool *pgxpool.Pool) *SuggestStore { return &SuggestStore{pool: pool} }
+
+// Search returns up to suggestLimit item name suggestions matching q using
+// pg_trgm similarity search.  Results are ordered by usage_count DESC so the
+// most-used names appear first.
+func (s *SuggestStore) Search(ctx context.Context, q string) ([]string, error) {
+	rows, err := s.pool.Query(ctx, `
+		SELECT name
+		FROM item_names
+		WHERE name % $1 OR name LIKE $2
+		ORDER BY usage_count DESC, last_used_at DESC
+		LIMIT $3`,
+		q, q+"%", suggestLimit,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("suggeststore: search: %w", err)
+	}
+	defer rows.Close()
+
+	var names []string
+	for rows.Next() {
+		var name string
+		if err := rows.Scan(&name); err != nil {
+			return nil, fmt.Errorf("suggeststore: scan name: %w", err)
+		}
+		names = append(names, name)
+	}
+	return names, rows.Err()
+}