Report a zero-length or missing database that starts empty (closes #290)

Audit of the configuration and state read from files. Three cases
carried on silently: a zero-length webhooker.db booted as a first
start without the "created a new, empty database" warning, and a
missing or zero-length per-webhook database was replaced by an empty
one with only an INFO line, losing that webhook's events and pending
deliveries unannounced. All three now log that warning with the
file's path; CreateDB, which makes a new webhook's file, does not.

The README now says how .env, the main, per-webhook and archive
databases treat an empty file, and that a damaged per-webhook database
fails that webhook alone.

Model: opus-5-5

internal/database/database.go

@@ -8,7 +8,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"io/fs"
 	"log/slog"
 	"os"
 	"path/filepath"
@@ -203,8 +202,7 @@ func (d *Database) connectTo(dataDir string) error {
 	// Checked before opening, which creates the file. A DATA_DIR that
 	// is unexpectedly empty -- its volume not mounted, say -- looks
 	// exactly like a first start, so a new database is a warning.
-	_, statErr := os.Stat(dbPath)
-	created := errors.Is(statErr, fs.ErrNotExist)
+	created := missingOrEmpty(dbPath)
 
 	// Opened through OpenSQLite so this handle carries the same WAL
 	// journaling, busy timeout, immediate-transaction locking, and pool