Move the archive files into their own package (closes #541)

The SQLite archive files the database target writes are now managed by
internal/archive: file names, rotation, the writer of each target,
pruning, renames, the gzipped JSON export, the idle sweep, and the
rename lock, which was Handlers.renameMu. The database target stays in
internal/delivery and writes through archive.Writers; the engine still
closes the writers when it stops, so the stop order is unchanged. The
handlers take the rename lock from archive.Archives.RenameLock, and
rename and evict through archive.Archives.

Exported names drop their Archive prefix in the new package. The tests
moved with the code; the two that check a delivery's recorded attempt
stay in delivery.

Model: opus-5-5

internal/handlers/webhook_edit.go

@@ -6,8 +6,8 @@ import (
 	"strconv"
 
 	"github.com/go-chi/chi"
+	"sneak.berlin/go/webhooker/internal/archive"
 	"sneak.berlin/go/webhooker/internal/database"
-	"sneak.berlin/go/webhooker/internal/delivery"
 )
 
 // HandleSourceEdit shows the form to edit a webhook.
@@ -59,8 +59,8 @@ func (h *Handlers) HandleSourceEditSubmit() http.HandlerFunc {
 
 		sourceID := chi.URLParam(r, "sourceID")
 
-		h.renameMu.Lock()
-		defer h.renameMu.Unlock()
+		h.archives.RenameLock().Lock()
+		defer h.archives.RenameLock().Unlock()
 
 		var webhook database.Webhook
 
@@ -130,7 +130,7 @@ func (h *Handlers) applyWebhookEdit(
 	edited.RetentionDays = retentionDays
 
 	// A new name renames the archive files before it is saved (see
-	// delivery.Engine.Rename). If either step fails, the same targets'
+	// archive.Writers.Rename). If either step fails, the same targets'
 	// archives go back to the name that is still stored, without
 	// reading the main database again.
 	targets, err := h.renameWebhookArchives(
@@ -150,7 +150,7 @@ func (h *Handlers) applyWebhookEdit(
 			)
 		}
 
-		if errors.Is(err, delivery.ErrArchiveNameTaken) {
+		if errors.Is(err, archive.ErrNameTaken) {
 			h.renderWebhookEdit(
 				w, r, webhook, name, description, retention,
 				"Not saved: "+err.Error()+