Replace bitcoin electrs/esplora with fulcrum + mempool stack

[Kukks]

Summary

Drops the legacy bitcoin-side electrs and esplora services in favour of fulcrum (modern Electrum protocol) and the mempool stack (esplora-compatible REST + UI). The Liquid services (electrs-liquid, esplora-liquid, chopsticks-liquid) are deliberately left untouched.

Architecture

                  ┌──> fulcrum (Electrum TCP 50000, WS 50003)
bitcoin (RPC) ────┤
                  └──> mempool-api (REST /api/*) <─┬── mempool-web (UI :5000)
                          ↑                        │
                          └── mempool-mariadb      │
                                                   │
chopsticks ──> esplora-relay (nginx :30000) ──[rewrite ^→/api]──┘

Why the nginx relay

chopsticks reverse-proxies its --electrs-addr verbatim with no path manipulation (verified by reading nigiri-chopsticks/router/electrs_handler.go). Mempool serves esplora-compat endpoints under /api/* while electrs served them at the root. The esplora-relay (nginx) sits between chopsticks and mempool-api, prepending /api so:

• nigiri faucet / mint / push CLI commands keep working
• arkd's ARKD_ESPLORA_URL: http://chopsticks:3000 keeps working with no env-var change

Port mapping (legacy host ports preserved)

Old (electrs/esplora)	New
30000 electrs HTTP esplora	esplora-relay (nginx) → mempool-api
50000 electrs Electrum TCP	fulcrum Electrum TCP
5000 esplora UI	mempool-web UI

New ports added: 50003 (fulcrum WS), 8999 (mempool-api).

Files touched

• cmd/nigiri/resources/docker-compose.yml — service swap
• cmd/nigiri/resources/esplora-relay.conf — new nginx config (path rewrite to /api/*)
• cmd/nigiri/main.go — embed + provision the new resource and per-service volume dirs
• cmd/nigiri/start.go — bitcoin explorer service list (CI / non-CI)
• internal/docker/mock_client.go — updated mock endpoints/ports
• test/start_stop_test.go — updated fixtures and expected service lists

Test plan

• go build ./...
• go vet ./...
• go test ./test/... -run 'TestDataDirSetup|TestBasicStartStop|TestServiceCombinations|TestStateManagement' — all green
• docker compose -f cmd/nigiri/resources/docker-compose.yml config --quiet — valid
• End-to-end smoke: nigiri start boots the new stack; nigiri faucet <addr> works (chopsticks → relay → mempool-api round-trip)
• nigiri start --liquid — liquid stack untouched, still works
• nigiri start --ark — arkd's ESPLORA_URL still resolves; wallet creation/funding flow works
• nigiri start --ci — headless explorer stack starts (no mempool-web)

Notes / follow-ups

• Existing nigiri datadirs need nigiri forget before upgrade so the new docker-compose.yml and the esplora-relay/default.conf resource are re-provisioned.
• Mempool's frontend is configured with BACKEND_MAINNET_HTTP_HOST=mempool-api — the UI will label things as mainnet but talk to the regtest backend. We can revisit this if a regtest-native UX is needed.
• STATISTICS_ENABLED and LIGHTNING_ENABLED are both set to false to keep the regtest stack lean; toggle if needed.