feat(mcp): let the connector address carry its own key

verdande could not be added to Claude at all. The documented setup —
POST /api/v1/mcp with an Authorization header — cannot be configured from
Claude's custom-connector dialog, which asks for a name, a URL and OAuth
client credentials and has no field for a bearer token. The instructions
described something the interface does not offer.

Worse, the obvious guess failed silently in the most misleading way
available: /mcp was not a route, so the SPA fallback answered 200 with a
page of HTML. The connector reported a successful connection and then
failed to parse the response, which reads as a broken client rather than
a missing endpoint.

So /mcp exists now and takes its credential in the query string, exactly
as the calendar feed does and for the same reason: a client that cannot
be told to send a header means the address is the credential. The
request logger records paths without their query strings, so the key does
not reach the log.

It is deliberately outside /api/v1 and accepts the key and nothing else.
Reading a session cookie here would make it a state-changing POST outside
the CSRF check — a cross-site request that acts as whoever is signed in —
and there is a test that fails if that ever becomes true.

The API path is unchanged for anything that can send a header. The tokens
page now shows the connector address next to the token it belongs to,
since that is the one moment the token exists.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/httpapi/server.go

@@ -90,6 +90,18 @@ func New(cfg *config.Config, db *store.DB, log *slog.Logger, web fs.FS) *Server
 	// authenticated by the token in the recipient address.
 	r.Post("/inbound/mail", s.handleInboundMail)
 
+	// MCP with the token in the query string, for clients that cannot send a
+	// header. Claude's custom-connector dialog takes a URL and nothing else, so
+	// /api/v1/mcp — which wants Authorization: Bearer — cannot be configured from
+	// it at all. Same reasoning as the calendar feed above: a client that cannot
+	// be told to send a header means the token in the URL is the credential.
+	//
+	// Outside /api/v1 deliberately, because it must not be reachable with a
+	// session cookie. A cookie-authenticated POST with no CSRF check is a
+	// cross-site request waiting to happen; handleMCPWithKey accepts the key and
+	// nothing else.
+	r.Post("/mcp", s.handleMCPWithKey)
+
 	// Google sends the browser here after consent. Outside /api/v1 because it is a
 	// top-level navigation rather than a fetch, but still behind the session — the
 	// tokens it exchanges belong to whoever is signed in.