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>

flake.nix

@@ -61,6 +61,8 @@
         # Dev-only (not needed in Docker, but fine to include)
         pytest
         pytest-xdist
+        pytest-playwright
+        requests
         ruff
         filelock
       ]);
@@ -84,8 +86,13 @@
           pkgs.sqlite
           pkgs.skopeo  # For pushing Docker images
           pkgs.lefthook  # Git hooks manager
+          pkgs.playwright-driver  # Browser binaries for e2e tests
         ];
 
+        # Playwright browser configuration for NixOS
+        PLAYWRIGHT_BROWSERS_PATH = "${pkgs.playwright-driver.browsers}";
+        PLAYWRIGHT_SKIP_BROWSER_DOWNLOAD = "1";
+
         shellHook = ''
           export PYTHONPATH="$PWD/src:$PYTHONPATH"
           export PATH="$PWD/bin:$PATH"