Store the routing table in the database (part of #122)

A new routes table holds one row per prefix and origin AS: the peer set
as the bitset's bytes, and the two kept routes, each with its peer's
number, AS path and time in seconds. A new route_peers table holds each
peer's number and address.

WriteRouteChanges writes one TakeChanges result in a transaction.
ReadRouteTable reads both tables into a new routing table, through the
new Table.Load, which records no change. It fails on a row that names a
peer number missing from route_peers. Nothing uses them yet.

Model: opus-5-5

internal/database/schema.sql

@@ -146,4 +146,33 @@ CREATE INDEX IF NOT EXISTS idx_live_routes_v6_origin_prefix ON live_routes_v6(or
 -- last_updated: the oldest and newest route times in /api/v1/stats and
 -- /status.json.
 CREATE INDEX IF NOT EXISTS idx_live_routes_v4_last_updated ON live_routes_v4(last_updated);
-CREATE INDEX IF NOT EXISTS idx_live_routes_v6_last_updated ON live_routes_v6(last_updated);
+CREATE INDEX IF NOT EXISTS idx_live_routes_v6_last_updated ON live_routes_v6(last_updated);
+
+-- The whole routing table, as internal/routetable keeps it in memory: one row
+-- for each prefix and origin AS that some peer carries. Startup reads every
+-- row, and the rows of the pairs that change are written back.
+CREATE TABLE IF NOT EXISTS routes (
+	prefix TEXT NOT NULL, -- in the text form Go's net/netip prints
+	origin_asn INTEGER NOT NULL,
+	-- The peers whose current route to the prefix has this origin AS: a set
+	-- of their numbers in route_peers, as the bitset package's MarshalBinary
+	-- writes it.
+	peers BLOB NOT NULL,
+	-- The newest of those routes and the one before it, the two kept in full.
+	-- Each has its peer's number in route_peers, its AS path as a JSON array,
+	-- and its time in seconds since 1970, UTC. A route that is not kept has
+	-- NULL in its three columns.
+	newest_peer INTEGER,
+	newest_as_path TEXT,
+	newest_time INTEGER,
+	older_peer INTEGER,
+	older_as_path TEXT,
+	older_time INTEGER,
+	PRIMARY KEY (prefix, origin_asn)
+) WITHOUT ROWID;
+
+-- The peers of the routes table, each with the number it has there.
+CREATE TABLE IF NOT EXISTS route_peers (
+	number INTEGER PRIMARY KEY,
+	address TEXT NOT NULL UNIQUE
+);