Apply review polish to the digest send path (post-v0.1.0:76)

Non-blocking items from the v76 reviewer pass. No redeploy needed — the box runs
v76 and its happy path is unaffected; these ride the next build:

- digest_mailer.send_digest: when Gmail is enabled but no sender resolves
  (CRM_DIGEST_SENDER unset and no admin email), raise NoTransport so the caller
  returns a clear 400 instead of a generic 502.
- gmail_send.send_via_gmail: wrap OSError/URLError (timeout/DNS) as a RuntimeError
  ("Gmail API unreachable: ...") to match the HTTPError handling; include the
  sender in the HTTPError message for debuggability.
- credentials.py: correct the now-stale GMAIL_COMPOSE_SCOPE comment (the digest
  mailer sends with this scope; only outreach drafts never send).
- test_gmail_send.py: add the HTTPError->RuntimeError branch, default_sender DB
  fallback (+None case + env override), and the send_digest SMTP-tag path.

19/19 backend tests green.

backend/digest_mailer.py

@@ -52,6 +52,11 @@ def send_digest(conn, to_addrs, subject, body, sender_email=None):
     if t == "gmail-dwd":
         from email_integration import gmail_send
         sender = sender_email or default_sender(conn)
+        if not sender:
+            # Gmail IS available but we have nobody to send as — a config gap, not a
+            # send failure. Surface it as NoTransport so the caller returns a clear 400.
+            raise NoTransport("Gmail is enabled but no sender address is set: "
+                              "set CRM_DIGEST_SENDER or give an active admin an email.")
         result = gmail_send.send_via_gmail(sender, to_addrs, subject, body, conn=conn)
         result["transport"] = "gmail-dwd"
         return result