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/webhook_db_manager_test.go

@@ -289,6 +289,75 @@ func TestWebhookDBManager_LazyCreation(t *testing.T) {
 	assert.True(t, mgr.DBExists(webhookID))
 }
 
+// A webhook's database is made by CreateDB along with the webhook. One
+// that GetDB finds missing or zero-length has lost the webhook's events
+// and pending deliveries, so the empty database made in its place is
+// logged as a warning naming the file
+// (https://git.eeqj.de/sneak/webhooker/issues/290). CreateDB, and
+// reopening a database that is there, log no such warning.
+func TestWebhookDBManager_LostDatabaseIsLogged(t *testing.T) {
+	t.Parallel()
+
+	const created = `level=WARN msg="created a new, empty database"`
+
+	open := func(
+		t *testing.T, prepare func(*database.WebhookDBManager, string),
+	) (string, string) {
+		t.Helper()
+
+		var logs bytes.Buffer
+
+		mgr := database.NewTestWebhookDBManagerWithLogger(
+			t.TempDir(),
+			slog.New(slog.NewTextHandler(&logs, nil)),
+		)
+
+		webhookID := uuid.New().String()
+		prepare(mgr, webhookID)
+
+		_, err := mgr.GetDB(webhookID)
+		require.NoError(t, err)
+		require.NoError(t, mgr.CloseAll())
+
+		return logs.String(),
+			" webhook_id=" + webhookID + " path=" + mgr.DBPath(webhookID)
+	}
+
+	t.Run("missing", func(t *testing.T) {
+		t.Parallel()
+
+		logs, fields := open(
+			t, func(*database.WebhookDBManager, string) {},
+		)
+		assert.Contains(t, logs, created+fields)
+	})
+
+	t.Run("zero-length", func(t *testing.T) {
+		t.Parallel()
+
+		logs, fields := open(
+			t, func(mgr *database.WebhookDBManager, webhookID string) {
+				require.NoError(t, os.WriteFile(
+					mgr.DBPath(webhookID), nil, database.SQLiteFilePerm,
+				))
+			},
+		)
+		assert.Contains(t, logs, created+fields)
+	})
+
+	t.Run("created with the webhook, then reopened", func(t *testing.T) {
+		t.Parallel()
+
+		logs, _ := open(
+			t, func(mgr *database.WebhookDBManager, webhookID string) {
+				require.NoError(t, mgr.CreateDB(webhookID))
+				require.NoError(t, mgr.CloseAll())
+			},
+		)
+		assert.NotContains(t, logs, created)
+	})
+}
+
 func TestWebhookDBManager_DeliveryWorkflow(t *testing.T) {
 	t.Parallel()