Serve /api/v1/stats counts from realtime in-memory counters (closes #27)

The stats endpoints recomputed every count with a COUNT(*) over each
table plus a MIN/MAX union scan on every request, which took the whole
timeout and returned 500 once the database passed a few GiB.

The row counts (ASNs, prefixes, peerings, peers, live routes) now live
in memory: seeded once at startup and adjusted on each write as routes
and records are inserted, updated and removed, so a read serves them
with no scan and no timer. A live-route upsert is an UPDATE, then an
INSERT only when no row matched, so its rows-affected count tells a new
route from a re-announced one. Oldest/newest timestamps read one row
from each end of the last_updated index, which also fixes the per-call
scan warning. The distinct-prefix histogram stays a single indexed
query.

JSON shape of /api/v1/stats and the status page is unchanged.

Model: opus-4-8

TODO.md

@@ -23,6 +23,10 @@ runs make check on main.
 
 # Completed Steps
 
+- 2026-09-22: realtime in-memory database statistics: counts seeded at
+  startup and adjusted on every write, oldest/newest route timestamps via
+  index-end lookups; `/api/v1/stats` no longer scans the tables (closes
+  #27)
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints,
   Makefile shims, README Entrypoints section
 - 2026-02-22: repo policy compliance: required policy files, .gitignore