refactor(voice): optimize recording pipeline and improve performance

Refactor the voice recording and playback systems to improve efficiency, reduce latency, and enhance Docker build performance.

- **Infrastructure**: Optimize Dockerfiles using build mounts for pnpm cache and reorder layers for better caching of dependencies and build tools.
- **Backend/Gateway**:
  - Refactor `broadcast` module to use a generic event-based system instead of hardcoded functions.
  - Simplify voice recording logic by merging metadata and segment management into a unified `segment.ts`.
  - Optimize audio downsampling in `streamSetup.ts` using `Int16Array` views for better performance.
  - Implement `withFallback` utility for more robust Redis/Database command execution.
- **Frontend**:
  - Optimize audio playback visualization using pre-computed level shapes and efficient RMS calculation.
  - Reduce latency in voice commands by prioritizing WebSocket communication over HTTP.
  - Improve base64 encoding efficiency in audio transmission.
- **General**:
  - Add default value for `ADMIN_PASSWORD` in shared config.
  - Fix Docker healthcheck to use `127.0.0.1` instead of `localhost`.

infra/docker/Dockerfile.backend

@@ -8,7 +8,7 @@ RUN npm install -g pnpm
 # Create non-root user
 RUN addgroup -S app && adduser -S -G app app
 
-# Copy workspace files
+# Copy dependency definition files first for better caching
 COPY pnpm-workspace.yaml .
 COPY pnpm-lock.yaml .
 COPY package.json .
@@ -22,14 +22,17 @@ COPY packages/shared ./packages/shared
 # Copy service
 COPY services/backend ./services/backend
 
-# Install dependencies
-RUN pnpm install --frozen-lockfile
+# Install dependencies with build cache
+RUN --mount=type=cache,id=pnpm-store,target=/root/.local/share/pnpm/store \
+    pnpm install --frozen-lockfile
 
 # Build shared workspace dependency first
-RUN pnpm --filter './packages/shared' run build
+RUN --mount=type=cache,id=pnpm-store,target=/root/.local/share/pnpm/store \
+    pnpm --filter './packages/shared' run build
 
 # Build backend
-RUN pnpm --filter './services/backend' run build
+RUN --mount=type=cache,id=pnpm-store,target=/root/.local/share/pnpm/store \
+    pnpm --filter './services/backend' run build
 
 # Own dist + node_modules by non-root user
 RUN chown -R app:app /app