fix(csrf): validate against the presented session, not the most recent (6c-3)

RequireCSRF looked up csrf_token by user_id ORDER BY created_at DESC LIMIT 1
— the newest session, not the one whose Bearer token was presented. Two
consequences: created_at's 1-second resolution made two same-second sessions
a coin-flip (phantom 403s), and conceptually it accepted "any valid CSRF for
this user" rather than the one bound to this session. Now queried by
token_hash = SHA256(bearer) AND user_id. Authenticate already located this
row + checked expiry, so the expiry clause is dropped. Signature unchanged.

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

delphi-backend/Source/PM.Session.pas

@@ -101,7 +101,7 @@ end;
 procedure RequireCSRF(ARequest: TIdHTTPRequestInfo;
   AResponse: TIdHTTPResponseInfo; AUserId: Integer);
 var
-  LSubmitted, LStored: string;
+  LSubmitted, LStored, LTokenHash: string;
   LQ: TFDQuery;
 begin
   if SameText(ARequest.Command, 'GET') then Exit;
@@ -113,6 +113,14 @@ begin
     raise ESessionRejected.Create('missing csrf');
   end;
 
+  // Bind the CSRF check to the session actually PRESENTED (token_hash), not
+  // "the user's most recent session" (§6c-3). The old ORDER BY created_at
+  // could read a different session's token — and created_at's 1-second
+  // resolution made two same-second sessions a coin-flip → phantom 403s.
+  // user_id stays as a filter (defense in depth). Expiry isn't re-checked:
+  // Authenticate always runs before RequireCSRF and already did it.
+  LTokenHash := SHA256Hex(ExtractBearerToken(ARequest));
+
   DB.Lock;
   try
     LQ := TFDQuery.Create(nil);
@@ -120,8 +128,8 @@ begin
       LQ.Connection := DB.Connection;
       LQ.SQL.Text :=
         'SELECT csrf_token FROM sessions ' +
-        'WHERE user_id = :uid AND expires_at > datetime(''now'') ' +
-        'ORDER BY created_at DESC LIMIT 1';
+        'WHERE token_hash = :th AND user_id = :uid';
+      LQ.ParamByName('th').AsString  := LTokenHash;
       LQ.ParamByName('uid').AsInteger := AUserId;
       LQ.Open;
       if LQ.IsEmpty then