Fix the origin-changes review findings

Hold the 100,000-row cap after a failed write's rows are put back, dropping and counting the oldest rows past it. Stop counts the unwritten origin changes. The prefix handler's per-write origin change limit is a field, so a test checks that one write leaves the rest. The server and README now say where origin changes come from. README, TODO and schema say 72 hours or 500 rows, whichever keeps more. The prefix page and TODO list the origin change columns and say what the peer count counts. The page takes its 5,000-row note from the constant.

Model: deepseek-v4-pro

TODO.md

@@ -56,13 +56,16 @@ 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-  announced or withdrawn, the origin AS and the peer count.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)
+- 2026-11-05: 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 (closes #161). The peer count counts the
+  peers of every origin AS. Each row shows:
+    - the time
+    - announced or withdrawn
+    - the origin AS
+    - the peer count
 - 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