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.go

@@ -0,0 +1,120 @@
+// Package sync provides synchronisation primitives for the mitbringsl backend.
+// The Hybrid Logical Clock (HLC) combines a physical wall-clock with a logical
+// counter so that every event gets a strictly monotonically increasing int64
+// timestamp that is compatible with client-generated HLC values.
+//
+// Encoding: the top 48 bits hold wall-clock milliseconds since the Unix epoch;
+// the bottom 16 bits hold a counter that disambiguates events within the same
+// millisecond.  This lets us store the full HLC value in a single BIGINT column
+// while still being orderable and human-readable.
+package sync
+
+import (
+	"sync"
+	"time"
+)
+
+const (
+	// counterBits is the number of bits reserved for the intra-ms counter.
+	counterBits = 16
+	// maxCounter is the maximum value the intra-ms counter can hold.
+	maxCounter = (1 << counterBits) - 1
+)
+
+// clock is the process-global HLC state. All exported functions are safe for
+// concurrent use via the embedded mutex.
+var clock struct {
+	mu   sync.Mutex
+	last int64
+}
+
+// Now returns the current HLC value without advancing the state.
+// Useful for initialisation checks; prefer Tick for actual event timestamps.
+func Now() int64 {
+	clock.mu.Lock()
+	defer clock.mu.Unlock()
+	return tick(clock.last)
+}
+
+// Tick advances the global HLC past the given external value (e.g. a
+// client-supplied hlc_ts) and returns the new value.  It is safe to call with
+// 0 when there is no external value to incorporate.
+//
+// Guarantees:
+//   - result > last (strictly monotonic)
+//   - result >= externalHLC (causal ordering preserved)
+func Tick(externalHLC int64) int64 {
+	clock.mu.Lock()
+	defer clock.mu.Unlock()
+
+	wall := wallMS()
+	prev := clock.last
+
+	// Determine the maximum physical time we're aware of.
+	maxWall := max3(wall, wallOf(prev), wallOf(externalHLC))
+
+	var counter int64
+	switch {
+	case maxWall == wallOf(prev) && maxWall == wallOf(externalHLC):
+		// Both are in the same ms bucket: take the larger counter + 1.
+		c := counterOf(prev)
+		if ec := counterOf(externalHLC); ec > c {
+			c = ec
+		}
+		counter = c + 1
+	case maxWall == wallOf(prev):
+		counter = counterOf(prev) + 1
+	case maxWall == wallOf(externalHLC):
+		counter = counterOf(externalHLC) + 1
+	default:
+		// Wall time advanced; reset counter.
+		counter = 0
+	}
+
+	if counter > maxCounter {
+		// Counter overflow: steal a millisecond from the future.
+		maxWall++
+		counter = 0
+	}
+
+	next := (maxWall << counterBits) | counter
+	clock.last = next
+	return next
+}
+
+// After reports whether a causally follows b (i.e. a > b as integers).
+func After(a, b int64) bool { return a > b }
+
+// wallMS returns the current Unix epoch time in milliseconds.
+func wallMS() int64 { return time.Now().UnixMilli() }
+
+// wallOf extracts the wall-clock ms from an HLC value.
+func wallOf(hlc int64) int64 { return hlc >> counterBits }
+
+// counterOf extracts the logical counter from an HLC value.
+func counterOf(hlc int64) int64 { return hlc & maxCounter }
+
+// tick is the internal (lock-held) Tick implementation against wall time only.
+func tick(last int64) int64 {
+	wall := wallMS()
+	maxWall := wallOf(last)
+	if wall > maxWall {
+		return wall << counterBits
+	}
+	counter := counterOf(last) + 1
+	if counter > maxCounter {
+		maxWall++
+		counter = 0
+	}
+	return (maxWall << counterBits) | counter
+}
+
+func max3(a, b, c int64) int64 {
+	if b > a {
+		a = b
+	}
+	if c > a {
+		a = c
+	}
+	return a
+}