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/retention.go

@@ -184,8 +184,8 @@ func (r *RetentionReaper) sweep(ctx context.Context) {
 
 		wh := webhooks[i]
 
-		// Nothing to reap if the per-webhook database has never
-		// been created.
+		// A missing database has nothing to reap. Restart recovery
+		// reports a lost one (see WebhookDBManager.GetDB).
 		if !r.dbManager.DBExists(wh.ID) {
 			continue
 		}