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/handlers_test.go

@@ -16,6 +16,7 @@ import (
 	"github.com/stretchr/testify/require"
 	"go.uber.org/fx"
 	"go.uber.org/fx/fxtest"
+	"sneak.berlin/go/webhooker/internal/archive"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
 	"sneak.berlin/go/webhooker/internal/delivery"
@@ -55,12 +56,14 @@ func (n *recordingNotifier) Tasks() []delivery.Task {
 	return out
 }
 
-// recordingArchives is a delivery.Archives that records what it
+// recordingArchives is an archive.Archives that records what it
 // was asked to do, so a test can prove that a deletion or rename
-// path reached the delivery engine. After FailRenames, every
+// path reached the archive writers. After FailRenames, every
 // rename of that target fails with the given error. After
 // BlockNextRename, the next rename is recorded and then waits.
+// Lock and Unlock are a plain mutex, as in archive.Writers.
 type recordingArchives struct {
+	renameMu       sync.Mutex
 	mu             sync.Mutex
 	evicted        []string
 	evictedTargets []string
@@ -73,10 +76,10 @@ type recordingArchives struct {
 // errInjectedRename is the failure a test hands FailRenames.
 var errInjectedRename = errors.New("injected rename failure")
 
-// errNameTaken is what the delivery engine returns when a file
+// errNameTaken is what the archive writers return when a file
 // already has an archive's new name, here archive-taken.db.
 var errNameTaken = fmt.Errorf(
-	"%w: archive-taken.db", delivery.ErrArchiveNameTaken,
+	"%w: archive-taken.db", archive.ErrNameTaken,
 )
 
 // archiveRename is one recorded Rename call.
@@ -86,6 +89,14 @@ type archiveRename struct {
 	TargetName  string
 }
 
+func (r *recordingArchives) Lock() {
+	r.renameMu.Lock()
+}
+
+func (r *recordingArchives) Unlock() {
+	r.renameMu.Unlock()
+}
+
 func (r *recordingArchives) EvictWebhook(webhookID string) {
 	r.mu.Lock()
 	defer r.mu.Unlock()
@@ -265,7 +276,7 @@ func newTestAppWithConfig(
 			func() *recordingArchives {
 				return &recordingArchives{}
 			},
-			func(r *recordingArchives) delivery.Archives {
+			func(r *recordingArchives) archive.Archives {
 				return r
 			},
 			func() *testCircuitBreakers {