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, the sweep prunes every file
and deletes a rotated file it leaves empty, Download exports every file
oldest first with each row's period, and the target list names the
current file and totals the size of all of them.

Model: opus-5-5

internal/delivery/export_test.go

@@ -493,23 +493,32 @@ 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
@@ -539,7 +548,7 @@ 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.