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.

Renaming a webhook or a target renames its archive files under the
archive writer's lock, before the new name is saved, and back again if
the save fails. 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/delivery/export_test.go

@@ -473,7 +473,7 @@ func NewTestCircuitBreaker(
 type ExportArchivedEvent = archivedEvent
 
 // ExportArchiveWriter wraps an archiveWriter so black-box tests
-// can exercise the per-webhook archive file mechanics.
+// can exercise the archive file mechanics.
 type ExportArchiveWriter struct {
 	w *archiveWriter
 }
@@ -548,6 +548,12 @@ func (e *ExportArchiveWriter) Evict() {
 	e.w.evict()
 }
 
+// Rename gives the archive file a new name in the same directory,
+// as a rename of the webhook or target does.
+func (e *ExportArchiveWriter) Rename(name string) error {
+	return e.w.rename(name)
+}
+
 // HandleOpen reports whether the writer currently holds an open
 // archive handle.
 func (e *ExportArchiveWriter) HandleOpen() bool {
@@ -567,16 +573,16 @@ func (e *ExportArchiveWriter) Same(
 }
 
 // ExportArchiveWriterFor returns the archive writer the registry
-// currently caches for a webhook, or nil when none is cached. It
-// never creates one, so a test can hold a reference to the very
-// writer an eviction is about to detach.
+// currently caches for a database target, or nil when none is
+// cached. It never creates one, so a test can hold a reference to
+// the very writer an eviction is about to detach.
 func (e *Engine) ExportArchiveWriterFor(
-	webhookID string,
+	targetID string,
 ) *ExportArchiveWriter {
 	e.dbTarget.mu.Lock()
 	defer e.dbTarget.mu.Unlock()
 
-	w, ok := e.dbTarget.writers[webhookID]
+	w, ok := e.dbTarget.writers[targetID]
 	if !ok {
 		return nil
 	}
@@ -585,26 +591,26 @@ func (e *Engine) ExportArchiveWriterFor(
 }
 
 // ExportHasArchiveWriter reports whether the database target
-// currently caches an archive writer for a webhook.
+// type currently caches an archive writer for a target.
 func (e *Engine) ExportHasArchiveWriter(
-	webhookID string,
+	targetID string,
 ) bool {
 	e.dbTarget.mu.Lock()
 	defer e.dbTarget.mu.Unlock()
 
-	_, ok := e.dbTarget.writers[webhookID]
+	_, ok := e.dbTarget.writers[targetID]
 
 	return ok
 }
 
 // ExportArchiveHandleOpen reports whether the cached archive
-// writer for a webhook holds an open database handle. It
+// writer for a target holds an open database handle. It
 // returns false when no writer is cached.
 func (e *Engine) ExportArchiveHandleOpen(
-	webhookID string,
+	targetID string,
 ) bool {
 	e.dbTarget.mu.Lock()
-	w, ok := e.dbTarget.writers[webhookID]
+	w, ok := e.dbTarget.writers[targetID]
 	e.dbTarget.mu.Unlock()
 
 	if !ok {
@@ -618,12 +624,12 @@ func (e *Engine) ExportArchiveHandleOpen(
 }
 
 // ExportEnsureArchiveWriter creates (if needed) and returns the
-// archive file path of the cached writer for a webhook, so a
+// archive file path of the cached writer for a target, so a
 // test can prime the registry the way a delivery would.
 func (e *Engine) ExportEnsureArchiveWriter(
-	webhookID string,
+	targetID string,
 ) (string, error) {
-	w, err := e.dbTarget.writerFor(webhookID)
+	w, err := e.dbTarget.writerFor(targetID)
 	if err != nil {
 		return "", err
 	}
@@ -631,14 +637,14 @@ func (e *Engine) ExportEnsureArchiveWriter(
 	return w.path, nil
 }
 
-// ExportSweepWriterFor takes a webhook's registry writer exactly
+// ExportSweepWriterFor takes a target's registry writer exactly
 // as the idle sweep does, reporting whether the sweep had to
 // create the entry. It lets a test drive the registry through the
 // sweep's own entry point instead of choreographing goroutines.
 func (e *Engine) ExportSweepWriterFor(
-	webhookID string,
+	targetID string,
 ) (*ExportArchiveWriter, bool, error) {
-	w, created, err := e.dbTarget.sweepWriterFor(webhookID)
+	w, created, err := e.dbTarget.sweepWriterFor(targetID)
 	if err != nil {
 		return nil, false, err
 	}
@@ -649,9 +655,9 @@ func (e *Engine) ExportSweepWriterFor(
 // ExportReleaseSweepWriter releases a sweep-created registry entry
 // exactly as a finished sweep does.
 func (e *Engine) ExportReleaseSweepWriter(
-	webhookID string, w *ExportArchiveWriter,
+	targetID string, w *ExportArchiveWriter,
 ) {
-	e.dbTarget.releaseSweepWriter(webhookID, w.w)
+	e.dbTarget.releaseSweepWriter(targetID, w.w)
 }
 
 // NewTestArchiveSweeper builds an ArchiveSweeper backed by the