fix(security): Phase 4 edge hardening + SSRF egress guard

Backend security fixes from the Phase 1 register / Phase 2 roadmap (PR1 + V-01):

- V-01 SSRF: new SafeHttpGuard validates outbound unsubscribe URLs (scheme allowlist
  + DNS-resolve-and-block private/loopback/link-local/ULA/metadata ranges), wired into
  UnsubscribeService; the "unsubscribe" HttpClient now disables auto-redirect so a
  validated external URL can't 3xx into an internal target. +33 unit tests.
- V-04: session cookie SecurePolicy=Always in non-dev (SameAsRequest in dev).
- V-06: UseExceptionHandler/ProblemDetails in prod; Cleanup/Unsubscribe no longer
  echo ex.Message to clients (logged server-side, generic message returned).
- V-08: ForwardedHeaders trusted only from configurable KnownNetworks (default private
  ranges) + ForwardLimit, instead of trusting any client. New ForwardedHeaders config.
- V-09: returnUrl validated with Url.IsLocalUrl (no open redirect via OAuth flow).
- V-10: SearchService clamps Page/PageSize (<=200); Analytics clamps take/days.
- V-11: baseline security headers (nosniff, X-Frame-Options DENY, Referrer-Policy,
  COOP) + HSTS in prod.
- V-13: /app/info discloses only devMode to anonymous callers unless dev mode is on.
- V-12: API container runs as non-root 'app' user (keys dir pre-owned).
- V-03: Postgres + API ports bound to 127.0.0.1; POSTGRES_PASSWORD now required (no
  weak default fallback).

API compatibility preserved (clamps not rejections; error-body shape changes only on
failure paths). No DB migrations. Build + all 33 unit tests green. V-15 (MailKit
NU1902) persists across versions and the SMTP path is default-off — tracked, not bumped.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/InboxIntel.Api/Dockerfile

@@ -16,5 +16,13 @@ RUN dotnet publish src/InboxIntel.Api/InboxIntel.Api.csproj -c Release -o /app/p
 FROM mcr.microsoft.com/dotnet/aspnet:8.0 AS runtime
 WORKDIR /app
 COPY --from=build /app/publish .
+
+# V-12: run as the non-root 'app' user shipped in the .NET 8 images. Pre-create the
+# DataProtection key directory owned by that user so the (initially empty) 'keys'
+# volume inherits app ownership on first mount and key persistence still works.
+# NOTE: an EXISTING root-owned keys volume must be recreated for this to take effect.
+RUN mkdir -p /keys && chown -R app:app /keys /app
+USER app
+
 EXPOSE 8080
 ENTRYPOINT ["dotnet", "InboxIntel.Api.dll"]