Fix tombstone bug in stats and add cost statistics to forms

Bug fix: Stats queries (eggs/day, feed/bird/day, etc.) were not excluding
tombstoned (deleted) events. Updated EventStore.list_events() to exclude
tombstoned events by default via LEFT JOIN, and updated direct SQL queries
in stats.py with the same tombstone exclusion.

New stats added:
- Harvest form: cost/egg (global, 30-day avg)
- Sell form: avg price/egg (30-day)
- Give feed form: cost/bird/day (global, 30-day avg)

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

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

tests/test_event_store.py

@@ -359,3 +359,66 @@ class TestTombstoneChecking:
         )
 
         assert event_store.is_tombstoned(event.id) is True
+
+    def test_list_events_excludes_tombstoned_by_default(self, migrated_db, event_store, now_utc):
+        """list_events excludes tombstoned events by default."""
+        event1 = event_store.append_event(
+            event_type=PRODUCT_COLLECTED,
+            ts_utc=now_utc,
+            actor="ppetru",
+            entity_refs={},
+            payload={"order": 1},
+        )
+        event2 = event_store.append_event(
+            event_type=PRODUCT_COLLECTED,
+            ts_utc=now_utc + 1000,
+            actor="ppetru",
+            entity_refs={},
+            payload={"order": 2},
+        )
+
+        # Tombstone event1
+        tombstone_id = generate_id()
+        migrated_db.execute(
+            """INSERT INTO event_tombstones (id, ts_utc, actor, target_event_id, reason)
+               VALUES (?, ?, ?, ?, ?)""",
+            (tombstone_id, now_utc + 2000, "admin", event1.id, "Test deletion"),
+        )
+
+        events = event_store.list_events()
+
+        assert len(events) == 1
+        assert events[0].id == event2.id
+
+    def test_list_events_includes_tombstoned_when_requested(
+        self, migrated_db, event_store, now_utc
+    ):
+        """list_events includes tombstoned events when include_tombstoned=True."""
+        event1 = event_store.append_event(
+            event_type=PRODUCT_COLLECTED,
+            ts_utc=now_utc,
+            actor="ppetru",
+            entity_refs={},
+            payload={"order": 1},
+        )
+        event2 = event_store.append_event(
+            event_type=PRODUCT_COLLECTED,
+            ts_utc=now_utc + 1000,
+            actor="ppetru",
+            entity_refs={},
+            payload={"order": 2},
+        )
+
+        # Tombstone event1
+        tombstone_id = generate_id()
+        migrated_db.execute(
+            """INSERT INTO event_tombstones (id, ts_utc, actor, target_event_id, reason)
+               VALUES (?, ?, ?, ?, ?)""",
+            (tombstone_id, now_utc + 2000, "admin", event1.id, "Test deletion"),
+        )
+
+        events = event_store.list_events(include_tombstoned=True)
+
+        assert len(events) == 2
+        assert events[0].id == event1.id
+        assert events[1].id == event2.id