Add env example and require Alpaca creds from env

- add .env.example with ingest/config defaults
- warn on missing Alpaca/Databento envs and remove hardcoded Alpaca defaults
- document .env setup in README
- allow .env.example to be tracked

.env.example

@@ -0,0 +1,45 @@
+NATS_URL=nats://localhost:4222
+CLICKHOUSE_URL=http://localhost:8123
+CLICKHOUSE_DATABASE=default
+
+# Options ingest
+INGEST_ADAPTER=alpaca
+ALPACA_KEY_ID=
+ALPACA_SECRET_KEY=
+ALPACA_REST_URL=https://data.alpaca.markets
+ALPACA_WS_BASE_URL=wss://stream.data.alpaca.markets/v1beta1
+ALPACA_FEED=indicative
+ALPACA_UNDERLYINGS=SPY
+ALPACA_STRIKES_PER_SIDE=8
+ALPACA_MAX_DTE_DAYS=30
+ALPACA_MONEYNESS_PCT=0.06
+ALPACA_MONEYNESS_FALLBACK_PCT=0.1
+ALPACA_MAX_QUOTES=200
+
+# Databento replay
+DATABENTO_API_KEY=
+DATABENTO_DATASET=OPRA.PILLAR
+DATABENTO_SCHEMA=trades
+DATABENTO_START=
+DATABENTO_END=
+DATABENTO_SYMBOLS=SPY.OPT
+DATABENTO_STYPE_IN=parent
+DATABENTO_STYPE_OUT=instrument_id
+DATABENTO_LIMIT=0
+DATABENTO_PRICE_SCALE=1
+DATABENTO_PYTHON_BIN=py/.venv/bin/python
+
+# IBKR adapter (options)
+IBKR_HOST=127.0.0.1
+IBKR_PORT=7497
+IBKR_CLIENT_ID=0
+IBKR_SYMBOL=SPY
+IBKR_EXPIRY=20250117
+IBKR_STRIKE=450
+IBKR_RIGHT=C
+IBKR_EXCHANGE=SMART
+IBKR_CURRENCY=USD
+IBKR_PYTHON_BIN=python3
+
+# Equities ingest
+EMIT_INTERVAL_MS=1000