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
lock renames run under, 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 lock, 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.Lock()
+		defer h.archives.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()+