Name each database target's archive for its webhook and target (closes #376)

Each database target now has its own archive file,
archive-WEBHOOKNAME-TARGETNAME-TARGETID.db, instead of one
archive-WEBHOOKID.db per webhook. delivery.ArchiveFileName builds the
name: each name is lowercased, keeps ASCII letters and digits, turns
every other run of characters into one dash, and is cut to 40
characters.

A change of webhook or target name renames its archive files under the
archive writer's lock, before the new name is saved, and back again if
the save fails. A rename never replaces a file: if one already has the
new name, the edit is refused. Deleting a target evicts only that
target's writer. Archive files are never deleted, and nothing looks for
files under the old name.

Model: opus-5-5

internal/handlers/target_edit.go

@@ -1,6 +1,7 @@
 package handlers
 
 import (
+	"errors"
 	"net/http"
 
 	"github.com/go-chi/chi"
@@ -150,11 +151,42 @@ func (h *Handlers) applyTargetEdit(
 		target.MaxRetries = retries
 	}
 
+	oldName := target.Name
 	target.Name = name
 	target.Config = configJSON
 
-	err = h.db.DB().Save(target).Error
+	// A new name renames the archive file before it is saved (see
+	// delivery.Engine.Rename). If either step fails, it goes back to
+	// the name that is still stored.
+	err = h.renameTargetArchive(target, webhook.Name, oldName, name)
+	if err == nil {
+		err = h.db.DB().Save(target).Error
+	}
+
 	if err != nil {
+		restoreErr := h.renameTargetArchive(
+			target, webhook.Name, name, oldName,
+		)
+		if restoreErr != nil {
+			h.log.Error(
+				"failed to rename archive back",
+				"target_id", target.ID,
+				"error", restoreErr,
+			)
+		}
+
+		if errors.Is(err, delivery.ErrArchiveNameTaken) {
+			http.Error(
+				w,
+				"Not saved: "+err.Error()+
+					". Move that file out of the data directory, "+
+					"then save again.",
+				http.StatusConflict,
+			)
+
+			return
+		}
+
 		h.serverError(w, r, "failed to update target", err)
 
 		return
@@ -165,6 +197,21 @@ func (h *Handlers) applyTargetEdit(
 	)
 }
 
+// renameTargetArchive renames a database target's archive file from
+// the target name oldName to newName. It does nothing when the name
+// is unchanged; other target types have no archive.
+func (h *Handlers) renameTargetArchive(
+	target *database.Target,
+	webhookName, oldName, newName string,
+) error {
+	if h.archives == nil || oldName == newName ||
+		target.Type != database.TargetTypeDatabase {
+		return nil
+	}
+
+	return h.archives.Rename(target.ID, webhookName, newName)
+}
+
 // renderTargetEdit renders the target edit page with an optional
 // error message.
 func (h *Handlers) renderTargetEdit(