fix: refuse a Redis URL on port zero

parts.port returns 0 rather than raising, since 0 is inside the range it checks,
so the URL reached .env and the worker and cache had nothing to connect to.

docsgpt/deploy/commands.py

@@ -216,12 +216,18 @@ def _redis_urls(base: str) -> dict[str, str]:
         # urlsplit raises on things like redis://[::1 ; that is a typo, not a crash.
         raise DeployError(f"the Redis URL {base!r} could not be read: {exc}") from exc
     try:
-        _ = parts.port  # a non-numeric or out-of-range port raises here, not when the URL is split
+        port = parts.port  # a non-numeric or out-of-range port raises here, not when the URL is split
     except ValueError as exc:
         raise DeployError(
             f"the Redis URL {base!r} has an unusable port: {exc}. Pass a URL like "
             "redis://host:6379 or redis://host:6379/5."
         ) from exc
+    if port == 0:
+        # urlsplit is happy with it, since 0 is inside the range, but nothing can connect to it.
+        raise DeployError(
+            f"the Redis URL {base!r} has an unusable port: 0. Pass a URL like "
+            "redis://host:6379 or redis://host:6379/5."
+        )
     if parts.scheme not in ("redis", "rediss"):
         raise DeployError(
             f"the Redis URL {base!r} should start with redis:// or rediss://, with any options as "