Fix: wire JSON logging, sensitive data masking, and settings into generated projects

[manavgup]

Problem

The README promises features that are incomplete or not wired into generated projects:

1. JSON logging — log_format="json" is accepted by FastStackConfig but silently ignored. Only console text output works. README says "JSON + console dual-format".
2. Sensitive data masking — masking.py has the code, FastStackConfig.sensitive_fields accepts the config, but nothing connects them.
3. Generated main.py ignores user settings — hardcodes FastStackConfig(app_version="0.1.0") instead of reading LOG_LEVEL from .env via settings.

Fix

• Wire log_format and sensitive_fields into StructuredLogger.setup()
• Add JSON formatter using python-json-logger (already a dependency)
• Wire masking filter into the logger when sensitive_fields is configured
• Update main.py.j2 to pass settings.log_level to FastStackConfig
• Add tests for JSON output and masking integration

Acceptance criteria

• log_format="json" produces JSON log output
• sensitive_fields=["password"] masks values in logs
• Generated main.py reads LOG_LEVEL from .env
• All existing tests still pass