US07-05: Deliver Backup and Operational Recovery (#92)

delivery_backlog/stories/US08-01-trusted-hosts-auth.md

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+# US08-01 — Make the Trust Boundary Configurable and Authenticated
+
+Epic: [E08](../E08-container-deployment.md)
+
+As an operator, I want to reach the application through my own hostname without
+weakening it, so a container behind a reverse proxy is as safe as the loopback
+deployment it replaces.
+
+## Context
+
+`photo_pipeline/api/security.py` refuses any request whose `Host` or `Origin` is not
+loopback. That check is the current stand-in for authentication: whoever can reach
+`127.0.0.1:8000` is the owner. Behind a proxy the hostname is no longer loopback, so
+relaxing the check without adding an authentication gate would publish the library.
+
+## Acceptance criteria
+
+- Allowed hosts and origins come from configuration (`PHOTO_PIPELINE_*`), default to
+  the current loopback set, and an unset configuration behaves exactly as today.
+- Whenever a non-loopback host is configured, startup requires an access secret and
+  refuses to serve without one; loopback-only deployments keep working with no secret.
+- The secret is exchanged for the existing session cookie and CSRF token through the
+  bootstrap endpoint; every protected route keeps its current session and CSRF
+  requirements unchanged.
+- Forwarded headers (`X-Forwarded-Proto`, `X-Forwarded-Host`) are honored only from a
+  configured trusted proxy and ignored otherwise, so a client cannot forge its origin.
+- Cookies are marked `Secure` when the effective external scheme is HTTPS.
+- Failed authentication is rate-limited and logged without the secret, the session id,
+  or any request body.
+- Health endpoints stay reachable without the secret; nothing else does.
+
+## Automated tests
+
+- Unit tests for host/origin evaluation across loopback default, configured host,
+  unconfigured host, forged forwarded headers, and trusted-proxy forwarded headers.
+- Integration tests: startup refusal without a secret, successful exchange, wrong
+  secret, replay of an old session, cross-site request, and unauthenticated access to
+  every route class.
+
+## Dependencies
+
+- US07-02