Feat: Add replay-gain to audio playback

The `RecordingView` struct now includes `gain_db` to represent
replay-gain. This value is read from the recording's metadata (`.json`
sidecar) and passed to the frontend.

The JavaScript in `case_recordings.html` uses this `data-gain-db`
attribute to apply replay-gain using the Web Audio API. This ensures
consistent playback loudness across different recordings. The
implementation uses a shared `AudioContext` to manage resources
efficiently and handles cases where the browser might not support
`AudioContext`.

Additionally, the audio recording on Wear OS now uses
`MediaRecorder.AudioSource.VOICE_RECOGNITION` instead of `MIC`. This
leverages platform noise and echo cancellation, aiming for a cleaner
signal before server-side gain adjustment.

server/templates/case_recordings.html

@@ -143,7 +143,7 @@ try {
 <div class="recording{% if rec.failed %} failed-row{% endif %}">
 <div class="recording-head">
 <time class="rec-time" datetime="{{ rec.recorded_at_iso }}">{{ rec.recorded_at_iso }}</time>
-<div class="player" data-src="/web/audio/{{ slug }}/{{ case_id }}/{{ rec.filename }}"{% match rec.duration_seconds %}{% when Some with (d) %} data-duration="{{ d }}"{% when None %}{% endmatch %}>
+<div class="player" data-src="/web/audio/{{ slug }}/{{ case_id }}/{{ rec.filename }}"{% match rec.duration_seconds %}{% when Some with (d) %} data-duration="{{ d }}"{% when None %}{% endmatch %}{% match rec.gain_db %}{% when Some with (g) %} data-gain-db="{{ g }}"{% when None %}{% endmatch %}>
 <button type="button" class="play" aria-label="Abspielen"></button>
 <input type="range" class="seek" min="0" max="1000" value="0" step="1" aria-label="Position">
 <span class="time">0:00 / 0:00</span>
@@ -247,6 +247,25 @@ try {
     return `${m}:${sec.toString().padStart(2, '0')}`;
   };
 
+  // Shared AudioContext for replay-gain. Lazy-created on the first
+  // player that actually needs gain (Safari requires user-gesture
+  // creation); shared across all players to avoid hitting the
+  // per-page context limit. `null` if the browser does not expose
+  // an AudioContext at all — falls back to the plain <audio> path.
+  let sharedCtx = null;
+  const ensureCtx = () => {
+    if (!sharedCtx) {
+      const Ctx = window.AudioContext || window.webkitAudioContext;
+      if (Ctx) sharedCtx = new Ctx();
+    }
+    if (sharedCtx && sharedCtx.state === 'suspended') {
+      // Some Safari builds keep the context suspended even when
+      // created from a user gesture. Resume is a no-op when running.
+      sharedCtx.resume();
+    }
+    return sharedCtx;
+  };
+
   document.querySelectorAll('.player').forEach(p => {
     const src = p.dataset.src;
     const btn = p.querySelector('.play');
@@ -287,6 +306,26 @@ try {
       if (audio) return audio;
       audio = new Audio(src);
       audio.preload = 'metadata';
+
+      // Replay-gain: route through Web Audio with a GainNode so the
+      // recording plays at the consistent target loudness regardless
+      // of which client recorded it. `createMediaElementSource`
+      // *redirects* the audio element's output through the graph —
+      // forgetting the `connect(ctx.destination)` would silence the
+      // player. Skipped entirely when gain is 0, missing, or
+      // negative (older meta sidecars, silent files, analyze errors)
+      // so the default <audio> path still works.
+      const gainDb = parseFloat(p.dataset.gainDb);
+      if (isFinite(gainDb) && gainDb > 0) {
+        const ctx = ensureCtx();
+        if (ctx) {
+          const mediaSrc = ctx.createMediaElementSource(audio);
+          const gainNode = ctx.createGain();
+          gainNode.gain.value = Math.pow(10, gainDb / 20);
+          mediaSrc.connect(gainNode).connect(ctx.destination);
+        }
+      }
+
       audio.addEventListener('loadedmetadata', () => {
         if (isFinite(audio.duration) && audio.duration > 0) duration = audio.duration;
         tryApplyPendingSeek();