Rotate a database target's archive monthly, daily or hourly (closes #379)

A database target's rotation (none, monthly, daily or hourly) puts the
UTC period of each event's receive time in its archive file name, so
each file holds exactly its period's events. It is on the new webhook
page, the add target form and the target edit form, and shown in the
target list.

Renames move every one of a target's files and move them back if one
fails. The sweep prunes every file, one at a time under the target's
lock, and deletes a rotated file it leaves empty. Download lists the
files, then opens one at a time, oldest first, finding each again under
the target's current names, and gives each row its period. The target
list names the current file and totals the size of all of them.

Model: opus-5-5

internal/delivery/export_test.go

@@ -501,23 +501,32 @@ func NewExportArchiveWriter(
 	return &ExportArchiveWriter{w: w}
 }
 
-// Write archives a row through the writer.
+// Write archives a row through the writer, into the file named
+// without a period.
 func (e *ExportArchiveWriter) Write(
 	row ExportArchivedEvent, expiry time.Duration,
 ) error {
-	return e.w.write(row, expiry)
+	return e.w.write(row, expiry, "")
+}
+
+// WritePeriod archives a row through the writer, into the file for
+// period.
+func (e *ExportArchiveWriter) WritePeriod(
+	row ExportArchivedEvent, expiry time.Duration, period string,
+) error {
+	return e.w.write(row, expiry, period)
 }
 
 // Open opens the archive file, pruning when expiry is positive.
 func (e *ExportArchiveWriter) Open(expiry time.Duration) error {
-	return e.w.open(expiry)
+	return e.w.open(e.w.path, expiry)
 }
 
 // Reopen closes and reopens the archive file.
 func (e *ExportArchiveWriter) Reopen(
 	expiry time.Duration,
 ) error {
-	return e.w.reopen(expiry)
+	return e.w.reopen(e.w.path, expiry)
 }
 
 // SetNow replaces the clock the writer measures its reopen
@@ -547,7 +556,7 @@ func (e *ExportArchiveWriter) Path() string {
 func (e *ExportArchiveWriter) OpenExisting(
 	expiry time.Duration,
 ) error {
-	return e.w.openMode(archiveModeExisting, expiry)
+	return e.w.openMode(e.w.path, archiveModeExisting, expiry)
 }
 
 // SweepExpired runs an idle sweep of the archive.