Allow retention_days of 0 to mean retain forever (closes #79)

RetentionDays carried gorm:"default:30", so GORM substituted 30 for a
zero value while building the insert. A webhook could therefore never
be configured to keep its events indefinitely: the reaper's
retain-forever branch existed but was unreachable from the normal
create and edit flows.

Introduce database.RetentionForeverDays = 365 * 1000 as the sentinel
for "retain forever" and a Webhook.BeforeSave hook that rewrites any
non-positive RetentionDays to it. The rewrite has to live in the hook
rather than at the call sites: GORM applies the column default while
converting the model to insert values, which happens after BeforeSave,
so anything later loses that race. Putting it on the model also means
a future call site, such as the planned REST API, cannot bypass it.

The reaper now skips a webhook when Webhook.RetainsForever reports
true, which recognises the sentinel and keeps honouring the old <= 0
values for rows written before it existed. Without this the sentinel,
being positive, would have produced a cutoff a thousand years in the
past and a DELETE matching nothing on every sweep.

Form handling is shared by create and edit through parseRetentionDays
so the two cannot drift: an empty field keeps the previous behaviour
(default on create, unchanged on edit), 0 is honoured, and an
unparseable or negative value is a 400 that re-renders the form rather
than a silently substituted default.

The retention inputs drop max="365". That cap was not cosmetic: the
edit form pre-fills the stored value, so a retain-forever webhook
rendered 365000 into an input capped at 365 and browser validation
would have blocked saving any edit to it. min becomes 0 with a hint
explaining what 0 does, and the list and detail views render a
RetentionLabel of "forever" instead of a raw day count.

The 30-day default is consolidated into database.DefaultRetentionDays,
referenced from the handler and from the create form's pre-filled
value, with a test asserting it agrees with the struct tag that cannot
reference it.

internal/handlers/source_management.go

@@ -25,6 +25,37 @@ type WebhookListItem struct {
 // errMissingURL signals that a required URL was not provided.
 var errMissingURL = errors.New("missing URL")
 
+// errInvalidRetention signals a retention_days form value that is not
+// a non-negative whole number.
+var errInvalidRetention = errors.New("invalid retention days")
+
+// retentionErrorMessage is what the create and edit forms show the user
+// when parseRetentionDays returns errInvalidRetention.
+const retentionErrorMessage = "Retention must be a whole number of " +
+	"days, or 0 to retain events forever."
+
+// parseRetentionDays interprets a retention_days form value.
+//
+// An empty value yields fallback, which lets the create path apply the
+// default and the edit path leave the stored value unchanged. A value
+// of 0 is returned as 0 and is rewritten to the retain-forever
+// sentinel by database.Webhook's BeforeSave hook, so no handler needs
+// to know the sentinel. Anything unparseable or negative is an error
+// rather than a silently substituted default.
+func parseRetentionDays(raw string, fallback int) (int, error) {
+	raw = strings.TrimSpace(raw)
+	if raw == "" {
+		return fallback, nil
+	}
+
+	v, err := strconv.Atoi(raw)
+	if err != nil || v < 0 {
+		return 0, errInvalidRetention
+	}
+
+	return v, nil
+}
+
 // EventWithDeliveries holds an event and its deliveries.
 type EventWithDeliveries struct {
 	database.Event
@@ -106,11 +137,20 @@ func (h *Handlers) buildWebhookListItems(
 // HandleSourceCreate shows the form to create a new webhook.
 func (h *Handlers) HandleSourceCreate() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		data := map[string]any{
-			tmplKeyError: "",
-		}
+		h.renderTemplate(
+			w, r, "sources_new.html", newSourceFormData(""),
+		)
+	}
+}
 
-		h.renderTemplate(w, r, "sources_new.html", data)
+// newSourceFormData builds the template data for the webhook creation
+// form, carrying the retention default so the pre-filled value comes
+// from database.DefaultRetentionDays rather than being a third
+// hardcoded copy of the same policy.
+func newSourceFormData(errMsg string) map[string]any {
+	return map[string]any{
+		tmplKeyError:           errMsg,
+		"DefaultRetentionDays": database.DefaultRetentionDays,
 	}
 }
 
@@ -145,23 +185,26 @@ func (h *Handlers) HandleSourceCreateSubmit() http.HandlerFunc {
 		retentionStr := r.FormValue("retention_days")
 
 		if name == "" {
-			data := map[string]any{
-				tmplKeyError: "Name is required",
-			}
-
 			w.WriteHeader(http.StatusBadRequest)
-			h.renderTemplate(w, r, "sources_new.html", data)
+			h.renderTemplate(
+				w, r, "sources_new.html",
+				newSourceFormData("Name is required"),
+			)
 
 			return
 		}
 
-		retentionDays := defaultRetentionDays
+		retentionDays, retErr := parseRetentionDays(
+			retentionStr, database.DefaultRetentionDays,
+		)
+		if retErr != nil {
+			w.WriteHeader(http.StatusBadRequest)
+			h.renderTemplate(
+				w, r, "sources_new.html",
+				newSourceFormData(retentionErrorMessage),
+			)
 
-		if retentionStr != "" {
-			v, convErr := strconv.Atoi(retentionStr)
-			if convErr == nil && v > 0 {
-				retentionDays = v
-			}
+			return
 		}
 
 		h.createWebhookWithEntrypoint(
@@ -428,7 +471,25 @@ func (h *Handlers) applyWebhookEdit(
 
 	webhook.Name = name
 	webhook.Description = r.FormValue("description")
-	h.parseRetention(r, webhook)
+
+	// An empty field falls back to the stored value, so submitting the
+	// form without touching retention leaves the policy alone.
+	retentionDays, retErr := parseRetentionDays(
+		r.FormValue("retention_days"), webhook.RetentionDays,
+	)
+	if retErr != nil {
+		data := map[string]any{
+			tmplKeyWebhook: *webhook,
+			tmplKeyError:   retentionErrorMessage,
+		}
+
+		w.WriteHeader(http.StatusBadRequest)
+		h.renderTemplate(w, r, "source_edit.html", data)
+
+		return
+	}
+
+	webhook.RetentionDays = retentionDays
 
 	err := h.db.DB().Save(webhook).Error
 	if err != nil {
@@ -442,23 +503,6 @@ func (h *Handlers) applyWebhookEdit(
 	)
 }
 
-// parseRetention parses and applies retention_days from the
-// form.
-func (h *Handlers) parseRetention(
-	r *http.Request,
-	webhook *database.Webhook,
-) {
-	retStr := r.FormValue("retention_days")
-	if retStr == "" {
-		return
-	}
-
-	v, err := strconv.Atoi(retStr)
-	if err == nil && v > 0 {
-		webhook.RetentionDays = v
-	}
-}
-
 // HandleSourceDelete handles webhook deletion.
 func (h *Handlers) HandleSourceDelete() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {