Add Docker support and fix benchmark/data pipeline bugs

Containerize the application with ROCm GPU support for AMD Radeon R9700:
- Add Dockerfile with PyTorch/PyG ROCm 5.6 wheels
- Add docker-compose.yml with dashboard, live-trading, and train services
- Add .dockerignore and .env.example for configuration

Fix benchmark script to use batch_size instead of num_stocks for variable
dimensions, and replace fragile partial model surgery with a standalone MLP
for memory estimation.

Fix data pipeline to skip dates with no next trading date instead of
fabricating zero returns.

Add slippage to paper broker, optional mark-to-market prices to broker
interface, and health check endpoint for container orchestration.

.env.example

@@ -0,0 +1,45 @@
+# =============================================================================
+# StockGNN R9700 — Environment Variables
+# =============================================================================
+# Copy this file to .env and fill in your API keys.
+# docker compose will automatically load variables from .env at runtime.
+# =============================================================================
+
+# -----------------------------------------------------------------------------
+# AMD ROCm GPU Settings
+# -----------------------------------------------------------------------------
+# Override the GFX architecture version if auto-detection fails.
+# Common values:
+#   gfx1030 (Navi 21 / RX 6900 XT / Radeon Pro W6800)
+#   gfx1100 (Navi 31 / RX 7900 XTX / Radeon Pro W7900)
+#   gfx1101 (Navi 31 / RX 7900 XT / Radeon Pro W7800)
+#   gfx1102 (Navi 32 / RX 7700 XT)
+# Default assumes gfx1030 for broad compatibility.
+# HSA_OVERRIDE_GFX_VERSION=10.3.0
+
+# -----------------------------------------------------------------------------
+# Data Provider API Keys
+# -----------------------------------------------------------------------------
+POLYGON_API_KEY=
+ALPHA_VANTAGE_API_KEY=
+
+# -----------------------------------------------------------------------------
+# Alternative Data (News & Social)
+# -----------------------------------------------------------------------------
+NEWS_API_KEY=
+TWITTER_BEARER_TOKEN=
+REDDIT_CLIENT_ID=
+REDDIT_CLIENT_SECRET=
+
+# -----------------------------------------------------------------------------
+# Interactive Brokers (if using IB gateway for live trading)
+# -----------------------------------------------------------------------------
+# IB_HOST=127.0.0.1
+# IB_PORT=7497
+# IB_CLIENT_ID=1
+
+# -----------------------------------------------------------------------------
+# Application Behaviour
+# -----------------------------------------------------------------------------
+# STOCKGNN_ENV=production
+# LOG_LEVEL=INFO