Add Playwright e2e test infrastructure

Set up browser-based end-to-end testing using pytest-playwright:
- Add playwright-driver and pytest-playwright to nix flake
- Configure PLAYWRIGHT_BROWSERS_PATH for NixOS compatibility
- Create ServerHarness to manage live server for tests
- Add smoke tests for health endpoint and page loading
- Exclude e2e tests from pre-commit hook (require special setup)

Run e2e tests with: pytest tests/e2e/ -v -n 0

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

tests/e2e/__init__.py

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+# ABOUTME: End-to-end test package for browser-based testing.
+# ABOUTME: Uses Playwright to test the full application stack.

tests/e2e/conftest.py

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+# ABOUTME: E2E test fixtures and server harness for Playwright tests.
+# ABOUTME: Provides a live server instance for browser-based testing.
+
+import os
+import random
+import subprocess
+import sys
+import time
+
+import pytest
+import requests
+
+from animaltrack.db import get_db
+from animaltrack.migrations import run_migrations
+from animaltrack.seeds import run_seeds
+
+
+class ServerHarness:
+    """Manages a live AnimalTrack server for e2e tests.
+
+    Starts the server as a subprocess with an isolated test database,
+    waits for it to be ready, and cleans up after tests complete.
+    """
+
+    def __init__(self, port: int):
+        self.port = port
+        self.url = f"http://127.0.0.1:{port}"
+        self.process = None
+
+    def start(self, db_path: str):
+        """Start the server with the given database."""
+        env = {
+            **os.environ,
+            "DB_PATH": db_path,
+            "DEV_MODE": "true",
+            "CSRF_SECRET": "e2e-test-csrf-secret-32chars!!",
+            "TRUSTED_PROXY_IPS": "127.0.0.1",
+        }
+        # Use sys.executable to ensure we use the same Python environment
+        self.process = subprocess.Popen(
+            [
+                sys.executable,
+                "-m",
+                "animaltrack.cli",
+                "serve",
+                "--port",
+                str(self.port),
+                "--host",
+                "127.0.0.1",
+            ],
+            env=env,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+        )
+        self._wait_for_ready()
+
+    def _wait_for_ready(self, timeout: float = 30.0):
+        """Poll /healthz until server is ready."""
+        start = time.time()
+        while time.time() - start < timeout:
+            try:
+                response = requests.get(f"{self.url}/healthz", timeout=1)
+                if response.ok:
+                    return
+            except requests.RequestException:
+                pass
+            time.sleep(0.1)
+        # If we get here, dump stderr for debugging
+        if self.process:
+            stderr = self.process.stderr.read() if self.process.stderr else b""
+            raise TimeoutError(
+                f"Server not ready after {timeout}s. stderr: {stderr.decode('utf-8', errors='replace')}"
+            )
+
+    def stop(self):
+        """Stop the server and clean up."""
+        if self.process:
+            self.process.terminate()
+            try:
+                self.process.wait(timeout=5)
+            except subprocess.TimeoutExpired:
+                self.process.kill()
+                self.process.wait()
+
+
+@pytest.fixture(scope="session")
+def e2e_db_path(tmp_path_factory):
+    """Create and migrate a fresh database for e2e tests.
+
+    Session-scoped so all e2e tests share the same database state.
+    """
+    temp_dir = tmp_path_factory.mktemp("e2e")
+    db_path = str(temp_dir / "animaltrack.db")
+
+    # Run migrations
+    run_migrations(db_path, "migrations", verbose=False)
+
+    # Seed with test data
+    db = get_db(db_path)
+    run_seeds(db)
+
+    return db_path
+
+
+@pytest.fixture(scope="session")
+def live_server(e2e_db_path):
+    """Start the server for the entire e2e test session.
+
+    Uses a random port in the 33660-33759 range to avoid conflicts
+    with other services or parallel test runs.
+    """
+    port = 33660 + random.randint(0, 99)
+    harness = ServerHarness(port)
+    harness.start(e2e_db_path)
+    yield harness
+    harness.stop()
+
+
+@pytest.fixture(scope="session")
+def base_url(live_server):
+    """Provide the base URL for the live server."""
+    return live_server.url

tests/e2e/test_smoke.py

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+# ABOUTME: Basic smoke tests to verify the e2e test setup works.
+# ABOUTME: Tests server startup, health endpoint, and page loading.
+
+import pytest
+import requests
+from playwright.sync_api import Page, expect
+
+pytestmark = pytest.mark.e2e
+
+
+def test_healthz_endpoint(live_server):
+    """Verify health endpoint returns OK."""
+    response = requests.get(f"{live_server.url}/healthz")
+    assert response.status_code == 200
+    assert response.text == "OK"
+
+
+def test_home_page_loads(page: Page, live_server):
+    """Verify the home page loads successfully."""
+    page.goto(live_server.url)
+    # Should see the page body
+    expect(page.locator("body")).to_be_visible()
+
+
+def test_animals_page_accessible(page: Page, live_server):
+    """Verify animals list page is accessible."""
+    page.goto(f"{live_server.url}/animals")
+    # Should see some content (exact content depends on seed data)
+    expect(page.locator("body")).to_be_visible()