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/test_gmail_send.py

@@ -5,9 +5,11 @@ No network: the credential provider and urllib.request.urlopen are monkeypatched
 Run directly or via backend/run_tests.py.
 """
 import base64
+import io
 import json
 import os
 import sys
+import urllib.error
 import urllib.request
 
 sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
@@ -153,6 +155,68 @@ def main():
         gmail_send.gmail_available = orig_avail
         smtp_send.smtp_configured = orig_smtp_cfg
 
+    # 7. HTTPError from Gmail is wrapped as RuntimeError (not raised raw)
+    orig_build = gmail_send._creds.build_provider
+    gmail_send._creds.build_provider = lambda factory: _Provider()
+    orig_open2 = urllib.request.urlopen
+
+    def raise_http(req, timeout=None):
+        raise urllib.error.HTTPError(req.full_url, 403, "Forbidden", {},
+                                     io.BytesIO(b'{"error":"denied"}'))
+
+    urllib.request.urlopen = raise_http
+    try:
+        try:
+            gmail_send.send_via_gmail("g@x", ["a@x"], "s", "b"); ok = False
+        except RuntimeError as e:
+            ok = "403" in str(e)
+        except urllib.error.HTTPError:
+            ok = False  # should have been wrapped, not raised raw
+    finally:
+        urllib.request.urlopen = orig_open2
+        gmail_send._creds.build_provider = orig_build
+    check(ok, "Gmail HTTPError wrapped as RuntimeError carrying the status")
+
+    # 8. default_sender: env wins; else DB first-admin; else None
+    class _FakeCur:
+        def __init__(self, row):
+            self._row = row
+
+        def fetchone(self):
+            return self._row
+
+    class _Conn:
+        def __init__(self, row):
+            self._row = row
+
+        def execute(self, *a, **k):
+            return _FakeCur(self._row)
+
+    os.environ.pop("CRM_DIGEST_SENDER", None)
+    check(digest_mailer.default_sender(_Conn({"email": "first@ten31.xyz"})) == "first@ten31.xyz",
+          "default_sender falls back to first active admin")
+    check(digest_mailer.default_sender(_Conn(None)) is None,
+          "default_sender returns None when no admin has an email")
+    os.environ["CRM_DIGEST_SENDER"] = "env@ten31.xyz"
+    check(digest_mailer.default_sender(_Conn({"email": "first@ten31.xyz"})) == "env@ten31.xyz",
+          "CRM_DIGEST_SENDER overrides the DB lookup")
+    os.environ.pop("CRM_DIGEST_SENDER", None)
+
+    # 9. send_digest tags transport 'smtp' on the fallback path
+    orig_avail2 = gmail_send.gmail_available
+    orig_smtp_cfg2 = smtp_send.smtp_configured
+    orig_smtp_send = smtp_send.send_email
+    try:
+        gmail_send.gmail_available = lambda: False
+        smtp_send.smtp_configured = lambda: True
+        smtp_send.send_email = lambda to, subj, body, **k: {"sent_to": to, "from": "f@x"}
+        res = digest_mailer.send_digest(None, ["a@x"], "S", "B")
+        check(res["transport"] == "smtp", "send_digest tags transport smtp on fallback")
+    finally:
+        gmail_send.gmail_available = orig_avail2
+        smtp_send.smtp_configured = orig_smtp_cfg2
+        smtp_send.send_email = orig_smtp_send
+
     print()
     if FAILS:
         print(f"FAILED ({len(FAILS)}):")