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/tasks/security-audit/TASK.md

@@ -0,0 +1,8 @@
+Run a full security audit of this Go codebase. For each finding rated
+HIGH or CRITICAL: spawn a sub-agent using Task to implement the fix
+across all affected files (models, handlers, migrations, templates,
+tests). Each sub-agent must: 1) write a failing test that reproduces the
+vulnerability, 2) implement the fix, 3) run `go test ./...` and `go vet
+./...` in a loop until all pass, 4) commit with a message referencing
+the finding ID. After all sub-agents complete, generate a summary of
+what was fixed and what needs manual review.