Add Playwright e2e tests for all 8 spec acceptance scenarios

Implement browser-based e2e tests covering:
- Tests 1-5: Stats progression (cohort, feed, eggs, moves, backdating)
- Test 6: Event viewing and deletion UI
- Test 7: Harvest outcomes with yield items
- Test 8: Optimistic lock selection validation

Includes page objects for reusable form interactions and fresh_db
fixtures for tests requiring isolated database state.

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

tests/e2e/pages/animals.py

@@ -0,0 +1,72 @@
+# ABOUTME: Page object for animal-related pages (cohort creation, registry).
+# ABOUTME: Encapsulates navigation and form interactions for animal management.
+
+from playwright.sync_api import Page, expect
+
+
+class AnimalsPage:
+    """Page object for animal management pages."""
+
+    def __init__(self, page: Page, base_url: str):
+        self.page = page
+        self.base_url = base_url
+
+    def goto_cohort_form(self):
+        """Navigate to the create cohort form."""
+        self.page.goto(f"{self.base_url}/actions/cohort")
+        expect(self.page.locator("body")).to_be_visible()
+
+    def create_cohort(
+        self,
+        *,
+        species: str,
+        location_name: str,
+        count: int,
+        life_stage: str,
+        sex: str,
+        origin: str = "purchased",
+        notes: str = "",
+    ):
+        """Fill and submit the create cohort form.
+
+        Args:
+            species: "duck" or "goose"
+            location_name: Human-readable location name (e.g., "Strip 1")
+            count: Number of animals
+            life_stage: "hatchling", "juvenile", or "adult"
+            sex: "unknown", "female", or "male"
+            origin: "hatched", "purchased", "rescued", or "unknown"
+            notes: Optional notes
+        """
+        self.goto_cohort_form()
+
+        # Fill form fields
+        self.page.select_option("#species", species)
+        self.page.select_option("#location_id", label=location_name)
+        self.page.fill("#count", str(count))
+        self.page.select_option("#life_stage", life_stage)
+        self.page.select_option("#sex", sex)
+        self.page.select_option("#origin", origin)
+
+        if notes:
+            self.page.fill("#notes", notes)
+
+        # Submit the form
+        self.page.click('button[type="submit"]')
+
+        # Wait for navigation/response
+        self.page.wait_for_load_state("networkidle")
+
+    def goto_registry(self, filter_str: str = ""):
+        """Navigate to the animal registry with optional filter."""
+        url = f"{self.base_url}/registry"
+        if filter_str:
+            url += f"?filter={filter_str}"
+        self.page.goto(url)
+        expect(self.page.locator("body")).to_be_visible()
+
+    def get_animal_count_in_registry(self) -> int:
+        """Get the count of animals currently displayed in registry."""
+        # Registry shows animal rows - count them
+        rows = self.page.locator("table tbody tr")
+        return rows.count()