feat: add event backdating with collapsible datetime picker

Add ability to specify custom date/time when recording events,
enabling historical data entry. Forms show "Now - Set custom date"
with a collapsible datetime picker that converts to milliseconds.

- Add event_datetime_field() component in templates/actions.py
- Add datetime picker to all event forms (cohort, hatch, outcome,
  tag, attrs, move, feed)
- Add _parse_ts_utc() helper to parse form timestamp or use current
- Add tests for backdating functionality

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

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

tests/test_web_actions.py

@@ -1552,3 +1552,134 @@ class TestStatusCorrectValidation:
         )
 
         assert resp.status_code == 403
+
+
+# =============================================================================
+# Backdating Tests
+# =============================================================================
+
+
+class TestParseTsUtcHelper:
+    """Tests for the _parse_ts_utc helper function."""
+
+    def test_parse_ts_utc_returns_current_for_none(self):
+        """Returns current time when value is None."""
+        import time
+
+        from animaltrack.web.routes.actions import _parse_ts_utc
+
+        before = int(time.time() * 1000)
+        result = _parse_ts_utc(None)
+        after = int(time.time() * 1000)
+
+        assert before <= result <= after
+
+    def test_parse_ts_utc_returns_current_for_zero(self):
+        """Returns current time when value is '0'."""
+        import time
+
+        from animaltrack.web.routes.actions import _parse_ts_utc
+
+        before = int(time.time() * 1000)
+        result = _parse_ts_utc("0")
+        after = int(time.time() * 1000)
+
+        assert before <= result <= after
+
+    def test_parse_ts_utc_returns_current_for_empty(self):
+        """Returns current time when value is empty string."""
+        import time
+
+        from animaltrack.web.routes.actions import _parse_ts_utc
+
+        before = int(time.time() * 1000)
+        result = _parse_ts_utc("")
+        after = int(time.time() * 1000)
+
+        assert before <= result <= after
+
+    def test_parse_ts_utc_returns_provided_value(self):
+        """Returns the provided timestamp when valid."""
+        from animaltrack.web.routes.actions import _parse_ts_utc
+
+        # Use a past timestamp
+        past_ts = 1700000000000  # Some time in 2023
+
+        result = _parse_ts_utc(str(past_ts))
+
+        assert result == past_ts
+
+    def test_parse_ts_utc_returns_current_for_invalid(self):
+        """Returns current time when value is invalid."""
+        import time
+
+        from animaltrack.web.routes.actions import _parse_ts_utc
+
+        before = int(time.time() * 1000)
+        result = _parse_ts_utc("not-a-number")
+        after = int(time.time() * 1000)
+
+        assert before <= result <= after
+
+
+class TestBackdatingCohort:
+    """Tests for backdating cohort creation."""
+
+    def test_cohort_uses_provided_timestamp(self, client, seeded_db, location_strip1_id):
+        """POST uses provided ts_utc for backdating."""
+        # Use a past timestamp (Feb 13, 2025 in ms)
+        backdated_ts = 1739404800000
+
+        resp = client.post(
+            "/actions/animal-cohort",
+            data={
+                "species": "duck",
+                "location_id": location_strip1_id,
+                "count": "2",
+                "life_stage": "adult",
+                "sex": "female",
+                "origin": "purchased",
+                "ts_utc": str(backdated_ts),
+                "nonce": "test-backdate-cohort-1",
+            },
+        )
+
+        assert resp.status_code == 200
+
+        # Verify the event was created with the backdated timestamp
+        event_row = seeded_db.execute(
+            "SELECT ts_utc FROM events WHERE type = 'AnimalCohortCreated' ORDER BY id DESC LIMIT 1"
+        ).fetchone()
+        assert event_row is not None
+        assert event_row[0] == backdated_ts
+
+    def test_cohort_uses_current_time_when_ts_utc_zero(self, client, seeded_db, location_strip1_id):
+        """POST uses current time when ts_utc is 0."""
+        import time
+
+        before = int(time.time() * 1000)
+
+        resp = client.post(
+            "/actions/animal-cohort",
+            data={
+                "species": "duck",
+                "location_id": location_strip1_id,
+                "count": "2",
+                "life_stage": "adult",
+                "sex": "female",
+                "origin": "purchased",
+                "ts_utc": "0",
+                "nonce": "test-backdate-cohort-2",
+            },
+        )
+
+        after = int(time.time() * 1000)
+
+        assert resp.status_code == 200
+
+        # Verify the event was created with a current timestamp
+        event_row = seeded_db.execute(
+            "SELECT ts_utc FROM events WHERE type = 'AnimalCohortCreated' ORDER BY id DESC LIMIT 1"
+        ).fetchone()
+        assert event_row is not None
+        assert before <= event_row[0] <= after