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/handlers/event_body.go

@@ -15,10 +15,12 @@ import (
 // eventBodyQuery reads one event's stored body as bytes. The cast
 // to blob is what makes the driver hand back the stored bytes
 // rather than a string conversion, so Content-Length taken from
-// the result matches what goes on the wire. The soft-delete
-// predicate is spelled out because Raw bypasses GORM's default
-// scope, and it is what stops a reaped event still being
-// downloadable.
+// the result matches what goes on the wire. The retention reaper
+// deletes event rows outright, so a reaped event is simply gone
+// and the query finds no row. The deleted_at predicate repeats
+// the soft-delete scope GORM adds to its own queries, which Raw
+// bypasses; nothing soft-deletes an event, so today it excludes
+// nothing.
 const eventBodyQuery = "SELECT cast(body as blob) " +
 	"FROM events WHERE id = ? AND webhook_id = ? AND deleted_at IS NULL"
 

internal/handlers/event_body_test.go

@@ -405,10 +405,11 @@ func TestHandleEventBodyDownload_UnknownEvent404s(t *testing.T) {
 // route. The body is read in one query before any header is
 // written, so a reaped event cannot produce a partial download:
 // it is a clean 404 with no Content-Length and no
-// Content-Disposition. Both removals the codebase performs are
-// covered — the reaper hard-deletes, and a soft-deleted row is
-// excluded by the query's own deleted_at predicate rather than
-// by GORM's default scope, which Raw bypasses.
+// Content-Disposition. The reaper deletes event rows outright,
+// which is the "hard deleted" case. The "soft deleted" case
+// covers a row no code produces today: it only pins the query's
+// own deleted_at predicate, the soft-delete condition Raw would
+// otherwise skip.
 func TestHandleEventBodyDownload_ReapedEvent404s(t *testing.T) {
 	t.Parallel()