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/delivery/engine_integration_test.go

@@ -1,6 +1,7 @@
 package delivery_test
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
@@ -1134,6 +1135,40 @@ 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),
+	)
+}
+
 // --- HTTP Config with custom headers ---
 
 func TestDeliverHTTP_CustomTargetHeaders(t *testing.T) {