fix: use sentinel value for optional brood location dropdown

FastHTML omits empty string attributes (value=""), causing browsers to
submit the option's text content "Same as hatch location" instead of an
empty value. This resulted in a ULID validation error.

Use "__none__" as a sentinel value that the server converts to None.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_web_actions.py

@@ -362,6 +362,34 @@ class TestHatchRecordingSuccess:
 
         assert count_at_nursery >= 3
 
+    def test_hatch_with_sentinel_brood_location_value(self, client, seeded_db, location_strip1_id):
+        """POST with __none__ sentinel value for brood location works correctly.
+
+        The form uses "__none__" as a sentinel value because FastHTML omits empty
+        string attributes, which would cause browsers to submit the option text
+        content instead.
+        """
+        resp = client.post(
+            "/actions/hatch-recorded",
+            data={
+                "species": "duck",
+                "location_id": location_strip1_id,
+                "assigned_brood_location_id": "__none__",
+                "hatched_live": "2",
+                "nonce": "test-hatch-nonce-sentinel",
+            },
+        )
+
+        assert resp.status_code == 200
+
+        # Verify hatchlings are at hatch location (not a separate brood location)
+        count_at_location = seeded_db.execute(
+            "SELECT COUNT(*) FROM animal_registry WHERE location_id = ? AND life_stage = 'hatchling'",
+            (location_strip1_id,),
+        ).fetchone()[0]
+
+        assert count_at_location >= 2
+
     def test_hatch_success_returns_toast(self, client, seeded_db, location_strip1_id):
         """Successful hatch recording renders toast in response body."""
         resp = client.post(