feat(dashboard-auth): cookie helpers for session_at/session_rt/pkce

Phase 3, Task 3.1. Three cookies:
  - hermes_session_at: OAuth access token (HttpOnly, TTL = token TTL)
  - hermes_session_rt: OAuth refresh token (HttpOnly, 30d max-age)
  - hermes_session_pkce: PKCE state + verifier + provider hint (10min)

All SameSite=Lax + Path=/. Secure flag is set ONLY when the request
scheme is https — uvicorn proxy_headers=True (enabled in gated mode at
Phase 3.5) rewrites scheme from X-Forwarded-Proto so Fly's TLS
terminator works.

hermes_cli/dashboard_auth/cookies.py

@@ -0,0 +1,120 @@
+"""Cookie helpers for dashboard auth.
+
+Three cookies in play:
+  - hermes_session_at: the OAuth access token
+                       (HttpOnly, lifetime = token TTL)
+  - hermes_session_rt: the OAuth refresh token
+                       (HttpOnly, lifetime = 30 days)
+  - hermes_session_pkce: short-lived PKCE state + CSRF nonce + provider
+                         hint (HttpOnly, lifetime = 10 minutes)
+
+All three are ``SameSite=Lax`` (browser will send on cross-site GET
+top-level navigation, which we need for the IDP redirect back to
+``/auth/callback``) and ``Path=/``. ``Secure`` is set ONLY when the
+dashboard was reached over HTTPS — detected via the request URL scheme,
+which honours ``X-Forwarded-Proto`` upstream of Fly's TLS terminator
+when uvicorn is configured with ``proxy_headers=True``. Loopback dev
+traffic is always HTTP so ``Secure`` would lock the cookies out of
+the browser.
+"""
+from __future__ import annotations
+
+from typing import Optional, Tuple
+
+from fastapi import Request
+from fastapi.responses import Response
+
+SESSION_AT_COOKIE = "hermes_session_at"
+SESSION_RT_COOKIE = "hermes_session_rt"
+PKCE_COOKIE = "hermes_session_pkce"
+
+# 30 days — matches Portal's REFRESH_TOKEN_TTL_SECONDS
+_RT_MAX_AGE = 30 * 24 * 60 * 60
+_PKCE_MAX_AGE = 10 * 60
+
+
+def _common_attrs(use_https: bool) -> dict:
+    attrs: dict = {
+        "httponly": True,
+        "samesite": "lax",
+        "path": "/",
+    }
+    if use_https:
+        attrs["secure"] = True
+    return attrs
+
+
+def set_session_cookies(
+    response: Response,
+    *,
+    access_token: str,
+    refresh_token: str,
+    access_token_expires_in: int,
+    use_https: bool,
+) -> None:
+    """Set both session cookies on the response.
+
+    ``access_token_expires_in`` is in seconds. Use the provider's reported
+    TTL for the access token. The refresh token cookie always lives 30
+    days regardless of the underlying provider's refresh TTL.
+    """
+    response.set_cookie(
+        SESSION_AT_COOKIE, access_token,
+        max_age=access_token_expires_in,
+        **_common_attrs(use_https),
+    )
+    response.set_cookie(
+        SESSION_RT_COOKIE, refresh_token,
+        max_age=_RT_MAX_AGE,
+        **_common_attrs(use_https),
+    )
+
+
+def clear_session_cookies(response: Response) -> None:
+    """Emit Max-Age=0 deletions for both session cookies."""
+    # Path must match the set-path for the delete to apply.
+    response.set_cookie(
+        SESSION_AT_COOKIE, "", max_age=0,
+        path="/", httponly=True, samesite="lax",
+    )
+    response.set_cookie(
+        SESSION_RT_COOKIE, "", max_age=0,
+        path="/", httponly=True, samesite="lax",
+    )
+
+
+def set_pkce_cookie(response: Response, *, payload: str, use_https: bool) -> None:
+    response.set_cookie(
+        PKCE_COOKIE, payload,
+        max_age=_PKCE_MAX_AGE,
+        **_common_attrs(use_https),
+    )
+
+
+def clear_pkce_cookie(response: Response) -> None:
+    response.set_cookie(
+        PKCE_COOKIE, "", max_age=0,
+        path="/", httponly=True, samesite="lax",
+    )
+
+
+def read_session_cookies(request: Request) -> Tuple[Optional[str], Optional[str]]:
+    """Returns (access_token, refresh_token), either may be None."""
+    at = request.cookies.get(SESSION_AT_COOKIE)
+    rt = request.cookies.get(SESSION_RT_COOKIE)
+    return at, rt
+
+
+def read_pkce_cookie(request: Request) -> Optional[str]:
+    return request.cookies.get(PKCE_COOKIE)
+
+
+def detect_https(request: Request) -> bool:
+    """Decide whether to set the ``Secure`` cookie flag.
+
+    Reads ``request.url.scheme`` — under uvicorn's ``proxy_headers=True``
+    (which start_server enables when the gate is active), this honours
+    ``X-Forwarded-Proto`` from Fly's TLS terminator. Loopback traffic is
+    always HTTP so this returns False there.
+    """
+    return request.url.scheme == "https"