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.go

@@ -98,34 +98,18 @@ func NewWebhookDBManager(
 	return m, nil
 }
 
-// GetDB returns the database connection for a webhook,
-// creating the database file lazily if it doesn't exist.
+// GetDB returns the database connection for a webhook, opening it on
+// first use.
+//
+// The file is made by CreateDB when the webhook is created. One that is
+// missing or zero-length here means the webhook's events and pending
+// deliveries are gone: an empty database is created in its place so
+// the webhook keeps receiving, and that is logged as a warning naming
+// the file, as a new main database is.
 func (m *WebhookDBManager) GetDB(
 	webhookID string,
 ) (*gorm.DB, error) {
-	// Fast path: already open
-	if val, ok := m.dbs.Load(webhookID); ok {
-		return asGormDB(val, webhookID)
-	}
-
-	// Slow path: open the database under the lock, looking in the
-	// cache again first. A caller that raced another one here then
-	// waits for its handle instead of opening a second one.
-	m.mu.Lock()
-	defer m.mu.Unlock()
-
-	if val, ok := m.dbs.Load(webhookID); ok {
-		return asGormDB(val, webhookID)
-	}
-
-	db, err := m.openDB(webhookID)
-	if err != nil {
-		return nil, err
-	}
-
-	m.dbs.Store(webhookID, db)
-
-	return db, nil
+	return m.getDB(webhookID, false)
 }
 
 // asGormDB returns a value read from the cache as the database
@@ -143,12 +127,12 @@ func asGormDB(val any, webhookID string) (*gorm.DB, error) {
 	return db, nil
 }
 
-// CreateDB explicitly creates a new per-webhook database file
-// and runs migrations.
+// CreateDB creates a new webhook's database file and runs
+// migrations.
 func (m *WebhookDBManager) CreateDB(
 	webhookID string,
 ) error {
-	_, err := m.GetDB(webhookID)
+	_, err := m.getDB(webhookID, true)
 
 	return err
 }
@@ -266,6 +250,48 @@ func (m *WebhookDBManager) DBPath(
 	return m.dbPath(webhookID)
 }
 
+// getDB is GetDB, and CreateDB when isNew is true: the webhook has just
+// been created, so a missing file is expected rather than lost.
+func (m *WebhookDBManager) getDB(
+	webhookID string, isNew bool,
+) (*gorm.DB, error) {
+	// Fast path: already open
+	if val, ok := m.dbs.Load(webhookID); ok {
+		return asGormDB(val, webhookID)
+	}
+
+	// Slow path: open the database under the lock, looking in the
+	// cache again first. A caller that raced another one here then
+	// waits for its handle instead of opening a second one.
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	if val, ok := m.dbs.Load(webhookID); ok {
+		return asGormDB(val, webhookID)
+	}
+
+	// Checked before opening, which creates the file. See GetDB.
+	path := m.dbPath(webhookID)
+	replaced := !isNew && missingOrEmpty(path)
+
+	db, err := m.openDB(webhookID)
+	if err != nil {
+		return nil, err
+	}
+
+	if replaced {
+		m.log.Warn(
+			"created a new, empty database",
+			"webhook_id", webhookID,
+			"path", path,
+		)
+	}
+
+	m.dbs.Store(webhookID, db)
+
+	return db, nil
+}
+
 func (m *WebhookDBManager) dbPath(
 	webhookID string,
 ) string {