Make live route writes cheaper so the prefix handler keeps up (closes #96)

The live route tables lose their random UUID key; the row id the
prefix-length sample draws is declared as the integer key. The prefix
tables are keyed by prefix and keep only first_seen; a batch inserts only
prefixes not stored yet. A route update sets only the columns that
change. Each kept index names the queries using it. Every write goes
through one connection kept for writing, with a 1 GiB page cache; reading
connections get 16 MiB, heap limits rise to 1.5 and 2 GiB (README
"Memory"). Batch writes go in key order. A benchmark run by hand writes
250,000 updates on 5 million routes faster than the feed's 10,000 a
second; a live run follows.

Model: opus-5-5

internal/database/slowquery.go

@@ -18,12 +18,12 @@ func logSlowQuery(logger *logger.Logger, query string, start time.Time) {
 	}
 }
 
-// exec wraps Exec with slow query logging
+// exec runs a statement on the writing connection, with slow query logging
 func (d *Database) exec(query string, args ...any) error {
 	start := time.Now()
 	defer logSlowQuery(d.logger, query, start)
 
-	_, err := d.db.ExecContext(context.Background(), query, args...)
+	_, err := d.writer.ExecContext(context.Background(), query, args...)
 
 	return err
 }