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>

.claude/skills/checkpoint/SKILL.md

@@ -0,0 +1,8 @@
+# Checkpoint Skill
+
+1. Run `go build ./...`  abort if errors
+2. Run `go test ./...`  abort if failures
+3. Run `go vet ./...`
+4. Run `git add -A && git status`  show user what will be committed
+5. Ask user for commit message
+6. Run `git commit -m "<message>"` and verify with `git log -1`