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
rename lock, 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 rename lock from archive.Archives.RenameLock, and
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.go

@@ -10,11 +10,11 @@ import (
 	"html/template"
 	"log/slog"
 	"net/http"
-	"sync"
 	"sync/atomic"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"go.uber.org/fx"
+	"sneak.berlin/go/webhooker/internal/archive"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/database"
 	"sneak.berlin/go/webhooker/internal/delivery"
@@ -65,7 +65,7 @@ type HandlersParams struct {
 	Session         *session.Session
 	Middleware      *middleware.Middleware
 	Notifier        delivery.Notifier
-	Archives        delivery.Archives
+	Archives        archive.Archives
 	CircuitBreakers delivery.CircuitBreakers
 	SSRFGuard       *delivery.Guard
 	Metrics         *metrics.Set
@@ -83,7 +83,7 @@ type Handlers struct {
 	session   *session.Session
 	mw        *middleware.Middleware
 	notifier  delivery.Notifier
-	archives  delivery.Archives
+	archives  archive.Archives
 	breakers  delivery.CircuitBreakers
 	mtr       *metrics.Set
 	templates map[string]*template.Template
@@ -93,17 +93,6 @@ 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. An archive download holds it while
-	// it reads the stored names and lists the files they give, and
-	// again for each file while it finds the file under the names
-	// stored then and opens it.
-	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.