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/internal/api/static/autoplay.js

@@ -0,0 +1,103 @@
+"use strict";
+(function () {
+    'use strict';
+    /* ── Background player ───────────────────────────────────── */
+    const bgAudio = new Audio();
+    let bgPlaying = false;
+    let bgBar = null;
+    let bgTitle = null;
+    let bgPlayBtn = null;
+    function createBgBar() {
+        var _a;
+        if (bgBar)
+            return;
+        bgBar = document.createElement('div');
+        bgBar.id = 'bg-bar';
+        bgBar.innerHTML =
+            '<span id="bg-title"></span>' +
+                '<button id="bg-play" aria-label="Abspielen">▶</button>' +
+                '<button id="bg-close" aria-label="Schließen">✕</button>';
+        document.body.appendChild(bgBar);
+        bgTitle = document.getElementById('bg-title');
+        bgPlayBtn = document.getElementById('bg-play');
+        bgPlayBtn.addEventListener('click', function () {
+            if (bgAudio.paused)
+                void bgAudio.play();
+            else
+                bgAudio.pause();
+        });
+        (_a = document.getElementById('bg-close')) === null || _a === void 0 ? void 0 : _a.addEventListener('click', function () {
+            bgAudio.pause();
+            if (bgBar)
+                bgBar.style.display = 'none';
+        });
+        bgAudio.addEventListener('play', function () { if (bgPlayBtn)
+            bgPlayBtn.textContent = '⏸'; });
+        bgAudio.addEventListener('pause', function () { if (bgPlayBtn)
+            bgPlayBtn.textContent = '▶'; });
+        bgAudio.addEventListener('ended', function () { if (bgPlayBtn)
+            bgPlayBtn.textContent = '▶'; });
+    }
+    function sendToBg(src, title) {
+        createBgBar();
+        if (bgBar)
+            bgBar.style.display = 'flex';
+        bgAudio.src = src;
+        if (bgTitle)
+            bgTitle.textContent = title;
+        void bgAudio.play();
+    }
+    // Attach "♪" button to every inline audio player
+    document.querySelectorAll('audio.media-audio').forEach(function (a) {
+        const btn = document.createElement('button');
+        btn.className = 'btn-bg-music';
+        btn.textContent = '♪ Hintergrundmusik';
+        btn.type = 'button';
+        const title = a.title || a.src.split('/').pop() || a.src;
+        btn.addEventListener('click', function () { sendToBg(a.src, title); });
+        a.insertAdjacentElement('afterend', btn);
+    });
+    /* ── Video autoplay + coordination ──────────────────────── */
+    const obs = new IntersectionObserver(function (entries) {
+        entries.forEach(function (e) {
+            const v = e.target;
+            if (e.isIntersecting) {
+                void v.play();
+            }
+            else {
+                v.pause();
+            }
+        });
+    }, { threshold: 0.3 });
+    document.querySelectorAll('video.media-embed').forEach(function (v) {
+        v.muted = true;
+        v.loop = true;
+        v.setAttribute('playsinline', '');
+        obs.observe(v);
+        // User unmutes → pause background music
+        v.addEventListener('volumechange', function () {
+            if (!v.muted && !v.paused) {
+                bgPlaying = !bgAudio.paused;
+                bgAudio.pause();
+            }
+            // Video muted again → resume background
+            if (v.muted && bgPlaying) {
+                void bgAudio.play();
+                bgPlaying = false;
+            }
+        });
+        // Video pauses or ends → resume background if it was playing
+        v.addEventListener('pause', function () {
+            if (bgPlaying) {
+                void bgAudio.play();
+                bgPlaying = false;
+            }
+        });
+        v.addEventListener('ended', function () {
+            if (bgPlaying) {
+                void bgAudio.play();
+                bgPlaying = false;
+            }
+        });
+    });
+})();