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

The routing table records a row each time a prefix and origin AS gets its first peer (announced) or loses its last (withdrawn), with the time and how many peers carry the prefix just after. The prefix handler writes the rows to a new origin_changes table, apart from routes. Every hour the database maintainer keeps each prefix's rows from the last 72 hours or its newest 500, whichever keeps more. The prefix page lists them newest first, at most 5,000.

At most 100,000 rows wait in memory to be written; past that the oldest are dropped and logged. A stop loses the rows past its last write.

Model: deepseek-v4-pro (implementation); opus-5-5 (commit message)

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 +56,19 @@ problem the run finds.
 
 # Completed Steps
 
+- 2026-11-05: the prefix page lists the prefix's origin changes, newest first.
+  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 or its rows from the last 72 hours, whichever keeps more. The
+  page shows at most the newest 5,000. Each row shows:
+    - the time
+    - announced or withdrawn
+    - the origin AS
+    - the peer count
+
+    A row's peer count is how many peers carry the prefix just after the change,
+    with any origin AS (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