feat: add API key authentication system for ChatGPT Business

Implements comprehensive Bearer token authentication to ensure only
authorized ChatGPT workspaces can access the MCP server.

Core Features:
- API key validation with constant-time comparison
- Multi-key support for rotation grace periods
- Rate limiting (5 failures per IP per 5 min)
- Comprehensive audit logging of all auth attempts
- IP-based failed attempt tracking

Key Management:
- generate_api_key.py: Create secure 64-char keys
- rotate_api_key.py: Guided key rotation with backup
- check_key_age.py: Automated expiration monitoring

Infrastructure:
- Traefik labels for HTTPS and rate limiting
- Security headers (HSTS, CSP, X-Frame-Options)
- Environment-based configuration
- Docker secrets support

Documentation:
- AUTH_SETUP.md: Complete authentication setup guide
- CHATGPT_SETUP.md: ChatGPT Business integration guide
- KEY_ROTATION.md: Key rotation procedures and automation

Security:
- Read-only operations enforced
- No write access to Gitea possible
- All auth attempts logged with correlation IDs
- Failed attempts trigger IP rate limits
- Keys never logged in full (only hints)

Breaking Changes:
- AUTH_ENABLED defaults to true
- MCP_API_KEYS environment variable now required
- Minimum key length: 32 characters (64 recommended)

Migration:
1. Generate API key: make generate-key
2. Add to .env: MCP_API_KEYS=<generated-key>
3. Restart: docker-compose restart aegis-mcp
4. Configure ChatGPT with Authorization header

Closes requirements for ChatGPT Business exclusive access.

Makefile

@@ -1,4 +1,4 @@
-.PHONY: help install install-dev test lint format clean build run docker-build docker-up docker-down docker-logs
+.PHONY: help install install-dev test lint format clean build run docker-build docker-up docker-down docker-logs generate-key rotate-key check-key-age
 
 help:
 	@echo "AegisGitea MCP - Available Commands"
@@ -11,6 +11,11 @@ help:
 	@echo "  make format         Format code with black"
 	@echo "  make clean          Remove build artifacts"
 	@echo ""
+	@echo "API Key Management:"
+	@echo "  make generate-key   Generate new API key"
+	@echo "  make rotate-key     Rotate existing API key"
+	@echo "  make check-key-age  Check API key age and expiration"
+	@echo ""
 	@echo "Local Execution:"
 	@echo "  make run            Run server locally (requires .env)"
 	@echo ""
@@ -19,6 +24,7 @@ help:
 	@echo "  make docker-up      Start containers"
 	@echo "  make docker-down    Stop containers"
 	@echo "  make docker-logs    View container logs"
+	@echo "  make docker-restart Restart MCP container"
 	@echo ""
 
 install:
@@ -67,3 +73,12 @@ docker-restart:
 
 docker-shell:
 	docker-compose exec aegis-mcp /bin/bash
+
+generate-key:
+	python3 scripts/generate_api_key.py
+
+rotate-key:
+	python3 scripts/rotate_api_key.py
+
+check-key-age:
+	python3 scripts/check_key_age.py