feat(PROJ-45): IMAP Per-Folder UID-Tracking + UIDVALIDITY-Check

- FolderState: GetFolderState, UpsertFolderState, ListFolderStates, DecideResync
- syncFolder nutzt per-folder UID-Tracking statt globalem highest_uid
- UIDVALIDITY-Check loest automatisch Full-Resync aus
- imap_folder_state Tabelle in initSchema (CREATE TABLE IF NOT EXISTS)
- SetAuditLogger in main.go verdrahtet

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/imap/store.go

@@ -86,6 +86,20 @@ ALTER TABLE imap_accounts ADD COLUMN IF NOT EXISTS sync_error_msg TEXT NOT NULL
 ALTER TABLE imap_accounts ADD COLUMN IF NOT EXISTS tenant_id INTEGER REFERENCES tenants(id);
 `
 
+// folderStateSQL creates the per-folder UID-tracking table introduced by PROJ-45.
+// Replaces the previous account-global last_uid tracking which silently skipped
+// folders with lower UIDs than INBOX and ignored UIDVALIDITY changes.
+const folderStateSQL = `
+CREATE TABLE IF NOT EXISTS imap_folder_state (
+	account_id   BIGINT NOT NULL REFERENCES imap_accounts(id) ON DELETE CASCADE,
+	folder       TEXT NOT NULL,
+	last_uid     BIGINT NOT NULL DEFAULT 0,
+	uid_validity BIGINT NOT NULL DEFAULT 0,
+	updated_at   TIMESTAMPTZ NOT NULL DEFAULT now(),
+	PRIMARY KEY (account_id, folder)
+);
+`
+
 // New creates a new Store, connects to PostgreSQL, and runs the migration.
 func New(dsn, secret string) (*Store, error) {
 	pool, err := pgxpool.New(context.Background(), dsn)
@@ -103,6 +117,11 @@ func New(dsn, secret string) (*Store, error) {
 		return nil, fmt.Errorf("imap store: migrate alter: %w", err)
 	}
 
+	if _, err := pool.Exec(context.Background(), folderStateSQL); err != nil {
+		pool.Close()
+		return nil, fmt.Errorf("imap store: migrate folder state: %w", err)
+	}
+
 	key := sha256.Sum256([]byte(secret))
 	return &Store{pool: pool, encKey: key}, nil
 }