config: add configurable RPC timeout for Ponder stability

Adds PONDER_RPC_TIMEOUT environment variable to improve Ponder
stability on slow RPC connections or under load. Default is 20000ms
(20 seconds).

## Changes
- containers/ponder-dev-entrypoint.sh: Export PONDER_RPC_TIMEOUT with default of 20000ms
- podman-compose.yml: Add PONDER_RPC_TIMEOUT to ponder service environment

## Configuration
The timeout can be overridden by setting PONDER_RPC_TIMEOUT in your
environment before starting the stack:
```bash
export PONDER_RPC_TIMEOUT=30000  # 30 seconds
./scripts/dev.sh start
```

## Impact
- Low risk: Configuration-only change
- No breaking changes
- Improves stability on slower networks or forked environments
- Defaults to 20 seconds if not specified

Fixes #87

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

containers/ponder-dev-entrypoint.sh

@@ -88,5 +88,6 @@ fi
 export CHOKIDAR_USEPOLLING=${CHOKIDAR_USEPOLLING:-1}
 export HOST=0.0.0.0
 export PORT=${PORT:-42069}
+export PONDER_RPC_TIMEOUT=${PONDER_RPC_TIMEOUT:-20000}
 
 exec npm run dev