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/handlers/handlers.go

@@ -99,7 +99,9 @@ type Handlers struct {
 	// 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 opens the file they give.
+	// 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