feat(auth): distinguish idle expiry from invalid refresh tokens

Second commit in the session-expiration-policy series. Lands the
error-detail taxonomy from §4.10 of the plan; no UI-visible change yet
because the frontend interceptor (commit 7) doesn't read the new detail
strings, but the wire is now ready for it.

Today every /auth/refresh failure returns 401 "Invalid refresh token"
regardless of cause, so the frontend has no way to distinguish "your
session ended for security" from "we don't recognize this token at
all." This commit introduces:

- decode_refresh_token_strict(): wraps jose.jwt.decode and raises a new
  IdleTokenExpired exception (from ExpiredSignatureError) so callers
  can branch on idle expiry. All other jose failures still propagate
  as JWTError. The legacy decode_token() is preserved for access-token,
  password-reset, and email-verification paths that don't need the
  distinction.
- get_refresh_token_payload(): now maps IdleTokenExpired ->
  "session_expired_idle", JWTError and wrong-type tokens ->
  "invalid_refresh_token".
- test_session_policy.py: new test file (will accumulate cases across
  the series). Three tests for the taxonomy: idle-expired returns
  session_expired_idle; wrong type returns invalid_refresh_token; bad
  signature returns invalid_refresh_token.

20/20 across test_session_policy + test_auth + test_oauth_callbacks.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/api/deps.py

@@ -7,7 +7,13 @@ from sqlalchemy import select
 import sentry_sdk
 
 from app.core.database import get_db
-from app.core.security import decode_token
+from jose import JWTError
+
+from app.core.security import (
+    IdleTokenExpired,
+    decode_refresh_token_strict,
+    decode_token,
+)
 from app.models.user import User
 from app.models.plan_limits import PlanLimits
 from app.core.tenant_context import set_current_account_id, clear_current_account_id
@@ -101,12 +107,35 @@ async def get_current_user_optional(
 async def get_refresh_token_payload(
     token: Annotated[str, Depends(oauth2_scheme)]
 ) -> dict:
-    """Extract and validate a refresh token from the Authorization header."""
-    payload = decode_token(token)
-    if payload is None or payload.get("type") != "refresh":
+    """Extract and validate a refresh token from the Authorization header.
+
+    Returns one of three outcomes via HTTP 401 `detail`:
+    - `session_expired_idle` — JWT signature valid but `exp` past
+    - `invalid_refresh_token` — any other decode failure, or `type != "refresh"`
+    - (200 path) — returns the decoded payload
+
+    The frontend uses these to choose between the "your session ended for
+    security" banner and a plain logout redirect. See
+    docs/plans/2026-05-13-session-expiration-policy.md §4.10.
+    """
+    try:
+        payload = decode_refresh_token_strict(token)
+    except IdleTokenExpired:
         raise HTTPException(
             status_code=status.HTTP_401_UNAUTHORIZED,
-            detail="Invalid refresh token",
+            detail="session_expired_idle",
+            headers={"WWW-Authenticate": "Bearer"},
+        )
+    except JWTError:
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="invalid_refresh_token",
+            headers={"WWW-Authenticate": "Bearer"},
+        )
+    if payload.get("type") != "refresh":
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="invalid_refresh_token",
             headers={"WWW-Authenticate": "Bearer"},
         )
     return payload