Bound the event log's rendered bodies in the query (closes #135)

The event log rendered stored bodies untruncated. Since buffered
rendering landed (#123) that became resident memory per concurrent
viewer, up to tens of MB, driven by payloads unauthenticated clients
supply to the public receiver.

Bound in the query rather than the template, via
substr(cast(body as blob), 1, ?) plus length(cast(body as blob)), so an
oversized body never becomes a Go string at all. Adds an EventLogView
projection carrying the true byte count, and trims a partial UTF-8 tail
without rewriting bodies that are merely invalid UTF-8.

Independently reviewed. The generated SQL was dumped under GORM DryRun
to confirm the cap is a bound parameter, both casts are present, and no
other path selects the full column; soft-delete scope, ordering and
pagination are unchanged.

Correction to the PR body: its quoted mutation output was produced by
removing the bound from eventLogColumns, not by raising the cap to
1<<30 as the text claimed. The reviewer reproduced the real
mutation and confirmed the tests do catch removal of the bound.

Follow-up #157 restores in-app retrieval of bodies above the cap.

internal/handlers/export_test.go

@@ -3,8 +3,35 @@ package handlers
 import (
 	"html/template"
 	"net/http"
+
+	"sneak.berlin/go/webhooker/internal/database"
 )
 
+// MaxRenderedBodyBytesForTest exposes the event log's body cap
+// to the handlers_test package.
+const MaxRenderedBodyBytesForTest = maxRenderedBodyBytes
+
+// TrimPartialRuneForTest exposes trimPartialRune for use in the
+// handlers_test package.
+func TrimPartialRuneForTest(b []byte) []byte {
+	return trimPartialRune(b)
+}
+
+// LoadEventLogViewsForTest exposes loadEventsWithDeliveries for
+// use in the handlers_test package. Assertions on the projected
+// body need the bytes as loaded: html/template rewrites invalid
+// UTF-8 on the way out, so the rendered page cannot show whether
+// a binary body survived the projection intact.
+func (s *Handlers) LoadEventLogViewsForTest(
+	w http.ResponseWriter,
+	webhook database.Webhook,
+	page int,
+) []EventLogView {
+	views, _ := s.loadEventsWithDeliveries(w, webhook, nil, page)
+
+	return views
+}
+
 // AddTemplateForTest registers a template under a page name so that
 // the handlers_test package can drive the render path with a
 // template of its own.