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

Audit of the files webhooker reads configuration or required state
from. A missing or zero-length database is reported with the "created
a new, empty database" warning and its path: webhooker.db at start,
and a per-webhook database at the latest at the next start, since
restart recovery now opens every webhook's database. The main
database's open errors name webhooker.db
(#459). webhooker resetpw
refuses a zero-length webhooker.db as it refuses a missing one. A
directory in place of a database file or its -wal or -shm is refused,
naming it; beside a -shm directory SQLite opened the database
read-only without a word. The README says how each case is treated.

Model: opus-5-5

internal/database/database_test.go

@@ -1,9 +1,15 @@
 package database_test
 
 import (
+	"bytes"
 	"context"
+	"log/slog"
+	"os"
+	"path/filepath"
 	"testing"
 
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 	"go.uber.org/fx/fxtest"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
@@ -100,3 +106,22 @@ func TestDatabaseConnection(t *testing.T) {
 		)
 	}
 }
+
+// TestOpen_UnreadableDatabaseIsNamed pins
+// https://git.eeqj.de/sneak/webhooker/issues/459: when SQLite cannot
+// read webhooker.db, the error that stops the server and `webhooker
+// resetpw` names the file, not only SQLite's own message.
+func TestOpen_UnreadableDatabaseIsNamed(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	path := filepath.Join(dir, database.MainDBFileName)
+	require.NoError(t, os.WriteFile(
+		path, bytes.Repeat([]byte("junk"), 1024), database.SQLiteFilePerm,
+	))
+
+	_, err := database.Open(dir, slog.New(slog.DiscardHandler))
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), path)
+	assert.Contains(t, err.Error(), "file is not a database")
+}