feat(pusher): add opt-in v2 WeTime ping signing

- Add WELIST_PING_SIG_VERSION (1 legacy, 2 canonical JSON); default 1
- Sign WELIST-PING-V2\n + compact sorted JSON covering url, chain, height,
  median_time, last_block_hash, info, base_fee, version and sig_version
- Fetch info/base_fee/version from the local /{chain}/info endpoint and sign them
- Fall back to null metadata on fetch failure; skip the fetch for onion URLs
- Normalize the advertised URL and bound metadata to match the WeTime service
- Activate v2 in bal-pusher.sh and document the TXT record setup
- Add unit tests for canonical payloads, URL normalization, metadata and onion detection

Includes previously uncommitted local deployment hardening:
- pg-server.sh configurable loopback bind address, volume and recreate handling
- .env loading in bal-server.sh and bal-pusher.sh
- second bal-server/bal-pusher development scripts

docs/03_architecture_and_data_flow.md

@@ -40,7 +40,7 @@ User
 4. **Monitoring**: The `bal-pusher` listens to the ZMQ `hashblock` topic with a 5-second receive timeout. When a new block is detected, it fetches blockchain info via RPC.
 5. **Evaluation**: The pusher queries the database for transactions with `status=0` and compares their locktime against the blockchain's best block height or median time (for timestamp-based locktimes above `LOCKTIME_THRESHOLD`).
 6. **Broadcast**: If the locktime is satisfied, the pusher sends the transaction via `sendrawtransaction` and updates the status to `1` (sent) or `2` (failed with error stored in `push_err`).
-7. **Statistics**: The pusher periodically calculates statistics and sends them to a remote `welist` server using an Ed25519-signed POST request. The server also exposes stats via the `GET /:network/stats` endpoint if `expose_stats` is enabled.
+7. **Statistics**: The pusher periodically calculates statistics and sends them to a remote `welist` server using an Ed25519-signed POST request. The signature scheme is selected by `WELIST_PING_SIG_VERSION`: legacy v1, or v2 over a canonical JSON payload whose `info`, `base_fee` and `version` come from the local `/{chain}/info` endpoint. The server also exposes stats via the `GET /:network/stats` endpoint if `expose_stats` is enabled.
 
 ## State Machine