Add Docker setup for BitcoinPurple explorer

- Dockerfile: node:20-alpine, compiles SCSS at build time, single image
  used for web server and all sync modes (blocks, peers, markets)
- docker-compose.yml: explorer + MongoDB (bind-mounted ./db) on shared
  'purple' network alongside bitcoinpurpled and electrumx
- docker/entrypoint.sh: generates settings.json from env vars via
  envsubst, dispatches to web/sync-blocks/sync-peers/reindex modes
- docker/settings.json.tmpl: minimal settings template parametrized
  for BitcoinPurple (coin, wallet RPC, MongoDB, theme)
- docker/mongo-init.sh: creates app user in explorerdb on first start
- .env.example: pre-filled defaults for BitcoinPurple
- CLAUDE.md: codebase guidance for Claude Code
- .gitignore: add db/ (MongoDB bind-mount data directory)

.env.example

@@ -0,0 +1,30 @@
+# ─── Coin ────────────────────────────────────────────────────────────────────
+COIN_NAME=BitcoinPurple
+COIN_SYMBOL=BTCP
+# POW, POS, or Hybrid
+COIN_DIFFICULTY=POW
+
+# ─── MongoDB (internal to this stack) ────────────────────────────────────────
+MONGO_HOST=mongodb
+MONGO_PORT=27017
+MONGO_USER=eiquidus
+MONGO_PASSWORD=change-me
+MONGO_DATABASE=explorerdb
+
+# ─── Wallet RPC ───────────────────────────────────────────────────────────────
+# bitcoinpurpled container is reachable on the shared 'purple' Docker network
+WALLET_RPC_HOST=bitcoinpurpled
+WALLET_RPC_PORT=13495
+# Copy rpcuser / rpcpassword values from ~/.bitcoinpurple/bitcoinpurple.conf
+WALLET_RPC_USER=
+WALLET_RPC_PASS=
+
+# ─── Explorer web ────────────────────────────────────────────────────────────
+# Host port the explorer is published on
+EXPLORER_PORT=3001
+# Bootswatch theme: Cerulean, Cosmo, Cyborg, Darkly, Flatly, Slate, Solar, ...
+EXPLORER_THEME=Darkly
+
+# ─── Sync intervals (seconds) ────────────────────────────────────────────────
+SYNC_BLOCKS_INTERVAL=120
+SYNC_PEERS_INTERVAL=300