fix(dht): make bittorrent-dht bootstrap reliably (1 → 50+ nodes)

[bluejorts]

Problem

Under Bun (the Docker image runtime), the DHT routing table never grows past a single node (itself), so Hydrabase peer discovery over the DHT is effectively non-functional. This reproduces on a bare host and in a bridge-networked container.

Root cause: usage, not the network

On the same machine and network, libtorrent (qBittorrent's engine) reaches 60–70 DHT nodes in ~20s, and a raw bittorrent-dht instance behaves identically under Node and Bun — so this is not a Bun/UDP problem and not a network/NAT problem. It's how the DHT is driven. Five compounding issues:

1. dht.listen() is never called. bittorrent-dht's internal bucket-refresh / re-bootstrap-when-isolated maintenance loop only starts on the socket listening event, which requires an explicit listen(). Without it, the library's own self-healing never runs and a cold start stalls after one shallow bootstrap lookup.
2. The 5s query timeout was ineffective. It was passed to krpc(), which ignores it; the socket used its 2s default and dropped slow-but-live routers.
3. Only a single bootstrap lookup ran, then the DHT idled until the 15-minute reannounce.
4. Low fan-out. k-rpc default concurrency/backgroundConcurrency (16/4).
5. Cache persistence never fired — gated on nodes > 50, which never happened, so restarts were always cold.

The bootstrapNodes list was also mostly dead: dht.libtorrent.org was on the wrong port (:6881 instead of :25401), several routers are defunct, and the list included Hydrabase peer hostnames that don't speak the BitTorrent DHT.

Fix (src/backend/networking/dht.ts, config.ts, krpc.d.ts)

• Call dht.listen() on a free ephemeral port (not node.port — the UTP transport owns that) to start the maintenance loop.
• Move the timeout to krpcSocket({ timeout: 5000 }) so it actually applies.
• Add a cold-bootstrap loop: re-seed routers and re-run lookup(ownId) every 10s until the table reaches 20 nodes.
• Raise concurrency 16→32 and backgroundConcurrency 4→16 (and add backgroundConcurrency to the KRPCOptions type).
• Persist the routing table on any new high ≥8 nodes (seeding cacheSize from the loaded cache) so restarts are warm.
• Replace the bootstrap list with empirically-verified live routers (probed with find_node): dht.transmissionbt.com:6881, dht.libtorrent.org:25401 (correct port), the canonical bittorrent/utorrent routers, and a transmissionbt IP fallback.

Verification

Environment	Before	After
Real app, host (bun run)	1 node	Ready with 76 nodes
Container, bridge networking	1 node	Ready with 41 nodes (climbing)

No dependency changes; the fix is self-contained to the DHT wiring, config, and one type declaration.

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