feat(api): sync réel — outbox POST /sync/ops idempotente + snapshot GET /sync/permissions (owner V1)

- operations.operation_id/ref_id BIGINT (= id client pending_operations), UNIQUE(device_id, operation_id) idempotente ; migration 000004 corrigée + test
- service/sync.go : ApplyBatch séquentiel (no-op si déjà traitée ; create déjà présent / update/move sur ressource absente / delete absent = no-op) ;
  arrêt à la première erreur non-idempotente (NAME_CONFLICT, NOT_FOUND, INVALID_REQUEST) + trace après succès ; ops share/share_link accusées sans état (V1)
- handlers/sync.go : POST /sync/ops → {applied, failed|null} ; GET /sync/permissions?after= → snapshot delta (effectiveAccess owner, cachedAt ms)
- snapshot : repo.ListOwned (delta updated_at en ms, trié) ; index partiel racine (owner_id, name) WHERE parent_id IS NULL
- GET /files sans folderId = racine uniquement (convention useFiles(undefined)) ; tests files_test corrigés (codes d'erreur imbriqués, tailles)
- fix dbtest : ping la connexion maintenance (création de la DB cible possible) — les tests handlers/repo tournent enfin contre PG réel
- tests handlers end-to-end : batch appliqué + retry idempotent, arrêt sur NAME_CONFLICT, delete idempotent, snapshot + delta

backend/repository/operations.go

@@ -0,0 +1,57 @@
+package repository
+
+import (
+	"database/sql"
+	"encoding/json"
+)
+
+// Operations persists the per-device outbox trace enforcing idempotence
+// UNIQUE(device_id, operation_id) — cf. docs/api-v1.md §6.1.
+type Operations struct {
+	DB *sql.DB
+}
+
+// Applied reports whether the operation was already processed for this device.
+func (o *Operations) Applied(deviceID string, operationID int64) (bool, error) {
+	var exists int
+	err := o.DB.QueryRow(
+		`SELECT 1 FROM operations WHERE device_id = $1 AND operation_id = $2`,
+		deviceID, operationID,
+	).Scan(&exists)
+	if err == sql.ErrNoRows {
+		return false, nil
+	}
+	return err == nil, err
+}
+
+// Record stores an applied operation trace (idempotent on replay).
+func (o *Operations) Record(deviceID string, operationID int64, opType, refType string, refID *int64, resourceID string, payload []byte) error {
+	var refTypeValue any
+	if refType != "" {
+		refTypeValue = refType
+	}
+	_, err := o.DB.Exec(
+		`INSERT INTO operations (device_id, operation_id, op_type, ref_type, ref_id, resource_id, payload)
+		 VALUES ($1, $2, $3, $4, $5, $6, $7)
+		 ON CONFLICT (device_id, operation_id) DO NOTHING`,
+		deviceID, operationID, opType, refTypeValue, refID, nullable(resourceID), jsonBytes(payload),
+	)
+	return err
+}
+
+func nullable(value string) any {
+	if value == "" {
+		return nil
+	}
+	return value
+}
+
+func jsonBytes(payload []byte) any {
+	if len(payload) == 0 || string(payload) == "null" {
+		return []byte(`{}`)
+	}
+	if !json.Valid(payload) {
+		return []byte(`{}`)
+	}
+	return payload
+}