feat: add comprehensive Playwright E2E test suite

- Create e2e/ directory with 7 test modules covering:
  - Home page: title, nav links, theme toggle, newsletter form
  - Cookie consent: accept all, reject all, granular prefs, persistence
  - Article index: loads, tag filter, click-through navigation
  - Article detail: title/read-time, share section, comments, newsletter aside, related
  - Comments: valid submit → redirect, empty body → error display, disabled check
  - Newsletter: JS confirmation message, invalid email error, aside form, duplicate
  - Feeds: RSS/sitemap/robots.txt validity, tag feed, seeded content present
- Extend seed_e2e_content management command with tagged article, about page,
  no-comments article, and legal pages for richer test coverage
- Add seed command tests (create + idempotency) to keep coverage ≥ 90%
- Add pr-e2e CI job (runs on pull_request): builds image, starts postgres + app,
  installs playwright, runs pytest e2e/
- Update nightly-e2e to run full e2e/ suite alongside legacy journey test
- Add --ignore=e2e to unit-test pytest step (coverage must not include browser tests)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

e2e/conftest.py

@@ -0,0 +1,40 @@
+"""Shared fixtures for E2E Playwright tests.
+
+All tests in this directory require a running application server pointed to by
+the E2E_BASE_URL environment variable.  Tests are automatically skipped when
+the variable is absent, making them safe to collect in any environment.
+"""
+
+from __future__ import annotations
+
+import os
+from collections.abc import Generator
+
+import pytest
+from playwright.sync_api import Browser, BrowserContext, Page, sync_playwright
+
+
+@pytest.fixture(scope="session")
+def base_url() -> str:
+    url = os.getenv("E2E_BASE_URL", "").rstrip("/")
+    if not url:
+        pytest.skip("E2E_BASE_URL not set – start a server and export E2E_BASE_URL to run E2E tests")
+    return url
+
+
+@pytest.fixture(scope="session")
+def _browser(base_url: str) -> Generator[Browser, None, None]:  # noqa: ARG001
+    """Session-scoped Chromium instance (headless)."""
+    with sync_playwright() as pw:
+        browser = pw.chromium.launch(headless=True)
+        yield browser
+        browser.close()
+
+
+@pytest.fixture()
+def page(_browser: Browser) -> Generator[Page, None, None]:
+    """Fresh browser context + page per test — no shared state between tests."""
+    ctx: BrowserContext = _browser.new_context()
+    pg: Page = ctx.new_page()
+    yield pg
+    ctx.close()