Rebuild PDF generation: HTML <table> + Playwright replaces PDFKit manual drawing

Task #372: Replaced the manual PDFKit coordinate-based PDF renderer with an
HTML table-based approach using Playwright's headless Chromium.

Key changes:
- Created `pdf-html-renderer.ts` with `buildScheduleHtml()` (generates full
  HTML with embedded base64 fonts, proper `<table>`, RTL/LTR support, row
  colors, merged cells, attendees formatting) and `htmlToPdfBuffer()` (renders
  HTML to PDF via Playwright Chromium).
- Updated `pdf-renderer.ts`: `renderSchedulePdf()` now delegates to the new
  HTML renderer via dynamic import. Legacy PDFKit code preserved as
  `renderSchedulePdf_LEGACY()` for fallback reference.
- Added `playwright-core` dependency and configured esbuild externals.
- Chromium binary discovery: searches `.cache/ms-playwright/` directories with
  env var override (`PLAYWRIGHT_CHROMIUM_EXECUTABLE_PATH`). Browser instance
  is cached as singleton with launch mutex to prevent concurrent double-launch.
- Process shutdown hook closes browser on exit.

Requirements met:
- Each meeting in exactly one `<tr>` with 4 `<td>` cells
- No text outside table, attendees ~3 per line inline
- Time always in its column, `page-break-inside: avoid`, `table-layout: fixed`
- Matches reference design (blue header, centered text, DIN Next LT Arabic)

artifacts/api-server/build.mjs

@@ -97,6 +97,7 @@ async function buildAll() {
       "zeromq",
       "zeromq-prebuilt",
       "playwright",
+      "playwright-core",
       "puppeteer",
       "puppeteer-core",
       "electron",