Add TypeScript migration, image resizing, media upload UX, and multimedia support

- Migrate static JS to TypeScript (static-ts/ → compiled to internal/api/static/)
- Add image resizing on upload: JPEG/PNG/WebP scaled to max 1920px at quality 80
- Extract shared upload logic into upload.go (saveUpload, saveResizedImage, saveResizedWebP)
- Add POST /media endpoint for drag-drop/paste media uploads with markdown ref return
- Add background music player with video/audio coordination (autoplay.ts)
- Add global nav, public feed, hashtags, visibility, Markdown rendering for entries
- Add Dockerfile stage for TypeScript compilation (static-ts-builder)
- Add goldmark, disintegration/imaging, golang.org/x/image dependencies

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/static-ts/tsconfig.json

@@ -0,0 +1,13 @@
+{
+  "compilerOptions": {
+    "target": "ES2017",
+    "lib": ["ES2017", "DOM"],
+    "strict": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "module": "None",
+    "ignoreDeprecations": "6.0",
+    "outDir": "../internal/api/static"
+  },
+  "include": ["./*.ts"]
+}