Add backdating support to egg harvest and sell forms

Both forms now have datetime pickers like the feed forms, allowing
users to record events at past timestamps. Each form has a unique
field_id (harvest_datetime, sell_datetime) to avoid conflicts.

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

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

src/animaltrack/web/routes/eggs.py

@@ -26,6 +26,26 @@ from animaltrack.services.products import ProductService, ValidationError
 from animaltrack.web.templates import render_page
 from animaltrack.web.templates.eggs import eggs_page
 
+
+def _parse_ts_utc(form_value: str | None) -> int:
+    """Parse ts_utc from form, defaulting to current time if empty or zero.
+
+    Args:
+        form_value: The ts_utc value from form data.
+
+    Returns:
+        Timestamp in milliseconds. Returns current time if form_value is
+        None, empty, or "0".
+    """
+    if not form_value or form_value == "0":
+        return int(time.time() * 1000)
+    try:
+        ts = int(form_value)
+        return ts if ts > 0 else int(time.time() * 1000)
+    except (ValueError, TypeError):
+        return int(time.time() * 1000)
+
+
 # APIRouter for multi-file route organization
 ar = APIRouter()
 
@@ -147,8 +167,8 @@ async def product_collected(request: Request, session):
             request, locations, products, location_id, "Quantity must be at least 1"
         )
 
-    # Get current timestamp
-    ts_utc = int(time.time() * 1000)
+    # Get timestamp - use provided or current (supports backdating)
+    ts_utc = _parse_ts_utc(form.get("ts_utc"))
 
     # Resolve ducks at location
     resolved_ids = resolve_ducks_at_location(db, location_id, ts_utc)
@@ -276,8 +296,8 @@ async def product_sold(request: Request, session):
             request, locations, products, product_code, "Total price cannot be negative"
         )
 
-    # Get current timestamp
-    ts_utc = int(time.time() * 1000)
+    # Get timestamp - use provided or current (supports backdating)
+    ts_utc = _parse_ts_utc(form.get("ts_utc"))
 
     # Create product service
     event_store = EventStore(db)

src/animaltrack/web/templates/eggs.py

@@ -16,6 +16,7 @@ from monsterui.all import (
 from ulid import ULID
 
 from animaltrack.models.reference import Location, Product
+from animaltrack.web.templates.actions import event_datetime_field
 
 
 def eggs_page(
@@ -163,6 +164,8 @@ def harvest_form(
             name="notes",
             placeholder="Optional notes",
         ),
+        # Event datetime picker (for backdating)
+        event_datetime_field("harvest_datetime"),
         # Hidden nonce for idempotency
         Hidden(name="nonce", value=str(ULID())),
         # Submit button
@@ -263,6 +266,8 @@ def sell_form(
             name="notes",
             placeholder="Optional notes",
         ),
+        # Event datetime picker (for backdating)
+        event_datetime_field("sell_datetime"),
         # Hidden nonce for idempotency
         Hidden(name="nonce", value=str(ULID())),
         # Submit button