[FEATURE] Add config file support for certificate pins

## Summary
Add runtime configuration file support for certificate pins to complement existing hardcoded defaults.

## Motivation
- Enable pin updates without recompilation in production
- Support emergency certificate rotations
- Allow environment-specific pins (dev/staging/prod)

## Proposed Solution
Implement hybrid approach in `CertificatePinner::initializeDefaultPins()`:
1. Try loading from `config/certificate_pins.json` first
2. Fall back to hardcoded pins if file doesn't exist
3. Maintain backward compatibility

## Implementation Notes
- Documentation already covers this approach (CERTIFICATE_PINNING.md:108-130)
- Keep hardcoded pins as safe defaults
- Add JSON schema validation for config file

## Phase
Phase 4 - Production Deployment

## References
- `core/utils/CertificatePinner.cpp:166-187`
- `docs/security/CERTIFICATE_PINNING.md`

Start with core/utils/CertificatePinner.cpp:166-187 and read CertificatePinner::initializeDefaultPins(). Review docs/security/CERTIFICATE_PINNING.md:108-130 for the documented configuration approach. Done means config/certificate_pins.json is supported with JSON schema validation, hardcoded pins remain the fallback, and existing behavior stays compatible.

Written by the indexing model from the issue text.