trusted proxy, TOTP replay protection, new tests

- Trusted proxy config option for proxy-aware IP extraction
  used by rate limiting and audit logs; validates proxy IP
  before trusting X-Forwarded-For / X-Real-IP headers
- TOTP replay protection via counter-based validation to
  reject reused codes within the same time step (±30s)
- RateLimit middleware updated to extract client IP from
  proxy headers without IP spoofing risk
- New tests for ClientIP proxy logic (spoofed headers,
  fallback) and extended rate-limit proxy coverage
- HTMX error banner script integrated into web UI base
- .gitignore updated for mciasdb build artifact

Security: resolves CRIT-01 (TOTP replay attack) and
DEF-03 (proxy-unaware rate limiting); gRPC TOTP
enrollment aligned with REST via StorePendingTOTP

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

dist/mcias-dev.conf.example

@@ -22,6 +22,7 @@ listen_addr = "127.0.0.1:8443"
 grpc_addr   = "127.0.0.1:9443"
 tls_cert    = "/tmp/mcias-dev.crt"
 tls_key     = "/tmp/mcias-dev.key"
+# trusted_proxy not set — direct local development, no reverse proxy.
 
 [database]
 path = "/tmp/mcias-dev.db"