PROXY-001: Add startup warning when TRUSTED_PROXY_SECRET empty in production
- validate_security_config() checks for missing proxy secret
- Additional warning when PKI enabled without proxy secret
- Tests for security configuration validation
BURN-001: HEAD requests now trigger burn-after-read deletion
- Prevents attacker from probing paste existence before retrieval
- Updated test to verify new behavior
RATE-001: Add RATE_LIMIT_MAX_ENTRIES to cap memory usage
- Default 10000 unique IPs tracked
- Prunes oldest entries when limit exceeded
- Protects against memory exhaustion DoS
Test count: 284 -> 291 (7 new security tests)
