Report or refuse each unusable file webhooker reads (closes #290)

An audit of every file webhooker reads configuration or required state from found cases that carried on silently. A zero-length webhooker.db, and a missing or zero-length per-webhook database, now log the "created a new, empty database" warning naming the file; restart recovery opens every live webhook's database, checking under the manager's lock that it still exists, so a missing one is reported at start. The main database's open errors name webhooker.db, for the server and webhooker resetpw; resetpw refuses a zero-length webhooker.db. A directory in place of any database file or its -wal or -shm is refused naming it. The README says how each case is treated. Also closes #459.

Model: opus-5-5

internal/database/sqlite_mode_test.go

@@ -182,6 +182,29 @@ func TestOpenSQLiteTightensFilesLeftWorldReadable(t *testing.T) {
 	requireDatabaseSetOwnerOnly(t, path)
 }
 
+// TestOpenSQLiteRefusesADirectorySidecar covers a directory in place
+// of -wal or -shm. Beside a -shm directory SQLite opens the database
+// read-only without a word, and every write then fails naming no file,
+// so the open must stop instead, naming the directory.
+func TestOpenSQLiteRefusesADirectorySidecar(t *testing.T) {
+	t.Parallel()
+
+	for _, suffix := range []string{"-wal", "-shm"} {
+		t.Run(suffix, func(t *testing.T) {
+			t.Parallel()
+
+			path := filepath.Join(t.TempDir(), database.MainDBFileName)
+			require.NoError(t, os.Mkdir(path+suffix, 0o700))
+
+			_, err := database.OpenSQLite(
+				path, database.SQLiteModeCreate,
+			)
+			require.Error(t, err)
+			assert.Contains(t, err.Error(), path+suffix)
+		})
+	}
+}
+
 // TestOpenSQLiteExistingModeDoesNotCreateTheFile guards the mechanism
 // the fix uses: OpenSQLite now creates the database file itself, and
 // must not do so for a caller that asked for an existing database. An