Serve the /api/v1/stats prefix distribution from memory (closes #30)

The prefix distribution was the last query on the stats path. It read one
index entry per live route on every request, so on a large database it took
the whole 4-second deadline and /api/v1/stats answered 500.

The distribution now lives next to the in-memory counts: seeded once at
startup from the same query, then kept exact by the live-route writes. A new
route whose prefix had no live route adds one at its mask length; a delete
that leaves a prefix with no live route takes one away. Each check is one
lookup on the prefix index, made only for new and removed routes, inside the
write.

Model: opus-5-5

TODO.md

@@ -24,10 +24,12 @@ https://git.eeqj.de/sneak/routewatch/pulls/6. After that, setting
 routewatch up under upaas on fsn1app1 and deploying it are his
 (https://git.eeqj.de/sneak/routewatch/issues/31), and so is the run under a
 real 5 GiB limit (https://git.eeqj.de/sneak/routewatch/issues/3).
-The other open issue is https://git.eeqj.de/sneak/routewatch/issues/30.
 
 # Completed Steps
 
+- 2026-09-29: `/api/v1/stats` serves the prefix distribution from memory,
+  seeded at startup and adjusted on every live-route write, so a request no
+  longer reads every live route (closes #30)
 - 2026-09-29: the entrypoint creates the data directory if it is missing and
   stops the start if a step fails; README "Running under upaas" no longer
   asks for the host directory to be created first (closes #42)
@@ -103,6 +105,3 @@ The other open issue is https://git.eeqj.de/sneak/routewatch/issues/30.
 - Production memory under 5 GiB: whether to test under a real 5 GiB
   container limit on fsn1app1 is open for sneak
   (https://git.eeqj.de/sneak/routewatch/issues/3)
-- `/api/v1/stats` answered HTTP 500 after 35 hours on the live feed, seen
-  on `3898daa`, which predates the 2026-09-22 in-memory statistics
-  (https://git.eeqj.de/sneak/routewatch/issues/30)