ciguard: Web UI is missing HTTP defence-in-depth headers

Summary

## Summary

ciguard's FastAPI Web UI (`src/ciguard/web/app.py`) does not set HTTP defence-in-depth headers. OWASP ZAP baseline scan flagged 11 alerts: missing Content-Security-Policy (Medium), X-Frame-Options (Medium), Sub-Resource-Integrity on `/api/docs` (Medium), COOP / COEP / CORP (Low), Permissions-Policy (Low), X-Content-Type-Options (Low).

## Threat scenario

For local-only deployment (current intent): minimal — there's no untrusted browser context, no third-party hosting, no auth surface to protect.

For public hosting (PRD Slice 9 GitHub App or hosted dashboard, future): each missing header reduces a defence layer:

- Missing CSP → injected XSS would have no second-line defence (first-line Jinja autoescape remains intact) - Missing X-Frame-Options → clickjacking against any UI button would be possible - Missing SRI on jsdelivr-hosted Swagger UI → if jsdelivr were compromised, attacker JS would run in the docs page context

## Patch

- New `SecurityHeadersMiddleware` at `src/ciguard/web/security_headers.py` injecting: X-Content-Type-Options nosniff, X-Frame-Options DENY, Referrer-Policy no-referrer, Permissions-Policy `interest-cohort=()`, Cross-Origin-Opener-Policy same-origin, Cross-Origin-Resource-Policy same-origin, plus per-path CSP with `/api/docs` + `/api/redoc` carve-out for cdn.jsdelivr.net (Swagger UI / ReDoc dependency). - COEP intentionally NOT set: would break Swagger UI's cross-origin assets, and ciguard makes no SharedArrayBuffer use that would benefit. - Registered via `app.add_middleware(SecurityHeadersMiddleware)`. - 6 regression tests in `tests/test_web.py::TestSecurityHeaders`.

## Discovery

Found by OWASP ZAP baseline scan during ciguard's first self-conducted pentest cycle, 2026-04-26.

## CVSS Scoring

- CVSS v3.1: `CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N` — 4.3 (Medium per v3.1 thresholds) - CVSS v4.0: `CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N` — first.org calc 4.3 (Low); GitHub's calc 2.1 (Low). All consistent at Low/borderline.

## Verification

``` $ curl -sI http://127.0.0.1:8080/ | grep -E '^(X-Frame|X-Content|Referrer|Permissions|Cross-Origin|Content-Security):' # Pre-fix: empty # Post-fix: 7 headers present ```

## Resources

- Fix released in [v0.8.2](https://github.com/Jo-Jo98/ciguard/releases/tag/v0.8.2) - CI regression gate added in [v0.8.3](https://github.com/Jo-Jo98/ciguard/releases/tag/v0.8.3)