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/delivery/engine_integration_test.go

@@ -1,6 +1,7 @@
 package delivery_test
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
@@ -1136,6 +1137,85 @@ func TestRecoverInFlight_WithPendingDeliveries(
 	}
 }
 
+// TestRecoverInFlight_ReportsAMissingWebhookDatabase covers a webhook
+// whose database file is gone, after a partial restore say. Restart
+// recovery opens every webhook's database, so the empty one made in its
+// place is reported at start, naming the file
+// (https://git.eeqj.de/sneak/webhooker/issues/290).
+func TestRecoverInFlight_ReportsAMissingWebhookDatabase(t *testing.T) {
+	t.Parallel()
+
+	mainDB := iMainDB(t)
+	webhookID := uuid.New().String()
+	iCreateWebhook(t, mainDB, webhookID, "lost-database")
+
+	var logs bytes.Buffer
+
+	dbMgr := database.NewTestWebhookDBManagerWithLogger(
+		t.TempDir(), slog.New(slog.NewTextHandler(&logs, nil)),
+	)
+	t.Cleanup(func() { _ = dbMgr.CloseAll() })
+
+	engine := delivery.NewTestEngineWithDB(
+		database.NewTestDatabase(mainDB), dbMgr,
+		slog.New(slog.DiscardHandler),
+		&http.Client{Timeout: 5 * time.Second}, 1,
+	)
+
+	engine.ExportRecoverInFlight(context.Background())
+
+	assert.Contains(
+		t, logs.String(),
+		`level=WARN msg="created a new, empty database" webhook_id=`+
+			webhookID+" path="+dbMgr.DBPath(webhookID),
+	)
+}
+
+// TestRecoverInFlight_SkipsAWebhookDeletedAfterTheListIsRead covers a
+// webhook deleted from the web interface while restart recovery runs.
+// Its database file is gone, and recovery must not create it again.
+func TestRecoverInFlight_SkipsAWebhookDeletedAfterTheListIsRead(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	mainDB := iMainDB(t)
+	webhookID := uuid.New().String()
+	iCreateWebhook(t, mainDB, webhookID, "deleted-during-recovery")
+
+	// The first query to return is recovery's read of the list of
+	// webhooks. Deleting the webhook right after it puts the delete
+	// between that read and the opening of the webhook's database.
+	deleted := false
+
+	require.NoError(t, mainDB.Callback().Query().After("gorm:query").
+		Register("delete-after-list", func(*gorm.DB) {
+			if deleted {
+				return
+			}
+
+			deleted = true
+
+			require.NoError(t, mainDB.Delete(
+				&database.Webhook{}, "id = ?", webhookID,
+			).Error)
+		}))
+
+	dbMgr := database.NewTestWebhookDBManager(t.TempDir())
+	t.Cleanup(func() { _ = dbMgr.CloseAll() })
+
+	engine := delivery.NewTestEngineWithDB(
+		database.NewTestDatabase(mainDB), dbMgr,
+		slog.New(slog.DiscardHandler),
+		&http.Client{Timeout: 5 * time.Second}, 1,
+	)
+
+	engine.ExportRecoverInFlight(context.Background())
+
+	require.True(t, deleted)
+	assert.False(t, dbMgr.DBExists(webhookID))
+}
+
 // --- HTTP Config with custom headers ---
 
 func TestDeliverHTTP_CustomTargetHeaders(t *testing.T) {