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_open.go

@@ -7,6 +7,7 @@ import (
 	"io/fs"
 	"net/url"
 	"os"
+	"syscall"
 	"time"
 
 	_ "modernc.org/sqlite" // Pure Go SQLite driver
@@ -93,7 +94,8 @@ const (
 const SQLiteFilePerm fs.FileMode = 0o600
 
 // reserveSQLiteFile puts path at SQLiteFilePerm before the driver ever
-// touches it, and tightens any sidecar already on disk.
+// touches it, and tightens any sidecar already on disk. A directory in
+// place of any of them is an error naming it.
 //
 // The mode has to be settled here rather than by a chmod after opening,
 // because SQLite picks it: robust_open substitutes
@@ -143,7 +145,15 @@ func reserveSQLiteFile(path string, create bool) error {
 	for _, p := range append(
 		[]string{path}, sqliteSidecarPaths(path)...,
 	) {
-		err := os.Chmod(p, SQLiteFilePerm)
+		// Chmod accepts a directory, and SQLite opens a database whose
+		// -shm is one read-only, without a word: every write then
+		// fails naming no file.
+		info, err := os.Stat(p)
+		if err == nil && info.IsDir() {
+			return fmt.Errorf("securing %s: %w", p, syscall.EISDIR)
+		}
+
+		err = os.Chmod(p, SQLiteFilePerm)
 		if err != nil && !errors.Is(err, fs.ErrNotExist) {
 			return fmt.Errorf("securing %s: %w", p, err)
 		}
@@ -152,6 +162,20 @@ func reserveSQLiteFile(path string, create bool) error {
 	return nil
 }
 
+// missingOrEmpty reports whether opening path in SQLiteModeCreate
+// would start a new, empty database: the file is not there, or it is
+// zero-length, which SQLite opens as an empty database. A file left at
+// zero length by an interrupted first start or a truncated copy holds
+// as little as a missing one, and must be reported the same way.
+func missingOrEmpty(path string) bool {
+	info, err := os.Stat(path)
+	if errors.Is(err, fs.ErrNotExist) {
+		return true
+	}
+
+	return err == nil && info.Size() == 0
+}
+
 // sqliteSidecarPaths returns the files SQLite maintains beside a
 // database under WAL. They carry the same rows as the database itself,
 // so a fix that tightens only the main file has fixed nothing.