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

Each database target now writes its own archive file, archive-WEBHOOKNAME-TARGETNAME-TARGETID.db, named by delivery.ArchiveFileName, in place of one archive per webhook keyed on its UUID. Renaming a webhook or a target renames its archive files (with any -wal and -shm) before the new name is saved, never over an existing file, and moves every one back if a rename or the save fails. The webhook edit, the target edit and target creation share one lock so no two interleave. Deleting a webhook or target leaves its files on disk. Nothing looks for the old archive-WEBHOOKID.db files. The README gives the naming and the recovery steps.

Model: opus-5-5

internal/handlers/handlers.go

@@ -10,6 +10,7 @@ import (
 	"html/template"
 	"log/slog"
 	"net/http"
+	"sync"
 	"sync/atomic"
 
 	"github.com/prometheus/client_golang/prometheus"
@@ -65,7 +66,7 @@ type HandlersParams struct {
 	Session      *session.Session
 	Middleware   *middleware.Middleware
 	Notifier     delivery.Notifier
-	Evictor      delivery.WebhookEvictor
+	Archives     delivery.Archives
 	SSRFGuard    *delivery.Guard
 	Metrics      *metrics.Set
 	Registry     *prometheus.Registry
@@ -82,7 +83,7 @@ type Handlers struct {
 	session   *session.Session
 	mw        *middleware.Middleware
 	notifier  delivery.Notifier
-	evictor   delivery.WebhookEvictor
+	archives  delivery.Archives
 	mtr       *metrics.Set
 	templates map[string]*template.Template
 
@@ -91,6 +92,14 @@ type Handlers struct {
 	// is one delivery will actually attempt.
 	ssrf *delivery.Guard
 
+	// renameMu makes the webhook edit, the target edit and target
+	// creation run one at a time, each held from loading the stored
+	// names through the archive rename, the save and any move back.
+	// Interleaved, one could rename an archive between another's
+	// rename and save, leaving the file named for one edit and the
+	// stored names from the other.
+	renameMu sync.Mutex
+
 	// dummyVerifications counts the equivalent-cost verifications
 	// charged for usernames that do not exist. It exists so a test
 	// can prove that path runs without measuring wall-clock time.
@@ -134,7 +143,7 @@ func New(
 	s.session = params.Session
 	s.mw = params.Middleware
 	s.notifier = params.Notifier
-	s.evictor = params.Evictor
+	s.archives = params.Archives
 	s.mtr = params.Metrics
 	s.ssrf = params.SSRFGuard