Release v0.27.0: security hardening (audit 0026)

Remediates the rfc-app application + deploy-config findings from the
Session 0026 security audit. Cut as the "v0.25.0-security-hardening"
branch (from v0.24.0); reversioned to 0.27.0 on rebase onto main since
v0.26.0 (#28) shipped while this was in flight.

- C1 (Critical): single sanitizeHtml.js chokepoint (DOMPurify) for every
  marked→innerHTML / dangerouslySetInnerHTML sink (MarkdownPreview,
  ProposalView x2, Editor); rel=noopener hook on target=_blank links.
- H1: per-account OTC-verify lockout (migration 023, auto-applied) +
  per-IP throttle via new ratelimit.py; wired on otc verify/request +
  passcode check/verify.
- M1: device_trust.lookup() single indexed-row read — cookie value is now
  "<row_id>.<raw_token>"; bcrypt-checks one row, not a global table scan.
  (Behavior change: existing device-trust cookies re-prompt once.)
- M2: HTTP security headers (CSP/HSTS/XFO/XCTO/Referrer-Policy) at nginx.
- M4: session cookie Secure-by-default (SESSION_COOKIE_SECURE opt-out).
- M5: bounce webhook fails CLOSED (503) when secret unset, instead of open;
  RFC_APP_INSECURE_BOUNCE_WEBHOOK=1 dev opt-in.
- L2/L3: per-IP cooldown + check-endpoint throttle.
- L4: systemd sandbox knobs. L8/I1: nginx server_tokens off + TLS1.0/1.1 out.

VERSION + frontend/package.json → 0.27.0; CHANGELOG documents the upgrade
steps (incl. the out-of-band nginx + systemd apply, which the flotilla
deploy gesture does not perform).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

deploy/systemd/rfc-app.service

@@ -42,5 +42,32 @@ ProtectHome=true
 PrivateTmp=true
 ReadWritePaths=/opt/rfc-app/backend/data
 
+# v0.25.0 security hardening (audit 0026 L4) — defense-in-depth.
+# The service binds 127.0.0.1:8000 and runs plain CPython
+# (FastAPI/uvicorn + sqlite + bcrypt + httpx), so it needs no
+# capabilities and no exotic syscalls.
+CapabilityBoundingSet=
+AmbientCapabilities=
+PrivateDevices=true
+ProtectKernelTunables=true
+ProtectKernelModules=true
+ProtectKernelLogs=true
+ProtectControlGroups=true
+RestrictAddressFamilies=AF_INET AF_INET6 AF_UNIX
+RestrictNamespaces=true
+LockPersonality=true
+# MemoryDenyWriteExecute=true blocks W^X memory — safe for stock
+# CPython (no JIT) and the pure-Python/C-extension stack here, but
+# would break a JIT or a C-ext that mmaps W+X. Watch the first
+# restart's journal for a crash; if uvicorn fails to come up,
+# comment this one line out and reload.
+MemoryDenyWriteExecute=true
+RestrictRealtime=true
+RestrictSUIDSGID=true
+SystemCallFilter=@system-service
+SystemCallErrorNumber=EPERM
+SystemCallArchitectures=native
+UMask=0077
+
 [Install]
 WantedBy=multi-user.target