Record origin changes and show them on the prefix page (closes #161)

The routing table records each prefix and origin AS pair's origin changes:
when it gains its first peer and when it loses its last. Each row holds the
prefix, the origin AS, announced or withdrawn, the peer count and the time.
The prefix handler writes them to a new origin_changes table, and the database
maintainer prunes each prefix to its newest 500 rows and 72 hours. The prefix
page lists them newest first, at most 5,000.

Model: deepseek-v4-pro

TODO.md

@@ -41,9 +41,8 @@ https://git.eeqj.de/sneak/routewatch/issues/225, also on `next`, takes the peers
 whose sessions go down together out of the table in one walk. A 24-hour run of
 `9bd0e91` on the live feed is checking whether any handler still drops updates.
 
-Two questions wait for sneak: which history the prefix page keeps
-(https://git.eeqj.de/sneak/routewatch/issues/161), and which fields the IP
-lookup answer gains (https://git.eeqj.de/sneak/routewatch/issues/203).
+One question waits for sneak: which fields the IP lookup answer gains
+(https://git.eeqj.de/sneak/routewatch/issues/203).
 
 The order of the project's remaining work is kept on
 https://git.eeqj.de/sneak/routewatch/issues/86.
@@ -57,6 +56,13 @@ problem the run finds.
 
 # Completed Steps
 
+- 2026-11-05: the prefix page lists the prefix's origin changes, newest first:
+  each time an origin AS gained its first peer or lost its last, with the time,
+  announced or withdrawn, the origin AS and the peer count. The routing table
+  records each change, and the prefix handler writes them to a new
+  `origin_changes` table. The database maintainer prunes each prefix to its
+  newest 500 rows and 72 hours. The page shows at most the newest 5,000 (closes
+  #161)
 - 2026-11-03: the streamer drops an update for a handler only when that
   handler's queue is full. It used to drop a random share of the updates once
   the queue was half full. The prefix handler's queue now holds 100,000