Add automatic GIF preview generation for videos

Backend changes:
- Add ffmpeg to Docker image for video processing
- Create gifGenerator utility with high-quality palette-based GIF generation
- Update Prisma schema: Add GIF to MediaKind enum
- Auto-generate 300px GIF preview (10fps, 3sec) on video upload
- Add POST /api/terms/:id/media/:mediaId/regenerate-gif endpoint for admins
- Update delete endpoint to cascade delete GIFs when video is deleted
- GIF generation uses ffmpeg with palette optimization for quality

Frontend changes:
- Update MediaKind enum to include GIF
- Display animated GIF previews in dictionary word cards
- Show 'No preview' icon for videos without GIF
- Add 'Regenerate GIF' button in admin video upload UI
- Display both video and GIF side-by-side in admin panel
- Visual indicator when GIF preview is available

Features:
- Automatic GIF generation on video upload (non-blocking)
- Manual GIF regeneration from admin panel
- GIF preview in dictionary listing for better UX
- Fallback to video icon when no GIF available
- Proper cleanup: GIFs deleted when parent video is deleted

Technical details:
- GIFs stored in /uploads/gifs/
- Uses two-pass ffmpeg encoding with palette for quality
- Configurable fps, width, duration, start time
- Error handling: video upload succeeds even if GIF fails

Co-Authored-By: Auggie

Dockerfile

@@ -0,0 +1,92 @@
+# ===========================================
+# Stage 1: Build Frontend
+# ===========================================
+FROM node:20-alpine AS frontend-builder
+
+WORKDIR /app
+
+# Copy workspace configuration files
+COPY package.json pnpm-lock.yaml pnpm-workspace.yaml ./
+COPY packages/frontend/package.json ./packages/frontend/
+
+# Install pnpm and dependencies
+RUN npm install -g pnpm && \
+    pnpm install --frozen-lockfile
+
+# Copy frontend source code
+COPY packages/frontend ./packages/frontend
+
+# Build frontend
+RUN cd packages/frontend && pnpm build
+
+# ===========================================
+# Stage 2: Build Backend
+# ===========================================
+FROM node:20-alpine AS backend-builder
+
+WORKDIR /app
+
+# Copy workspace configuration files
+COPY package.json pnpm-lock.yaml pnpm-workspace.yaml ./
+COPY packages/backend/package.json ./packages/backend/
+
+# Install pnpm and dependencies
+RUN npm install -g pnpm && \
+    pnpm install --frozen-lockfile
+
+# Copy backend source code
+COPY packages/backend ./packages/backend
+
+# Generate Prisma client and build backend
+RUN cd packages/backend && \
+    npx prisma generate && \
+    pnpm build
+
+# ===========================================
+# Stage 3: Production Runtime
+# ===========================================
+FROM node:20-alpine AS production
+
+WORKDIR /app
+
+# Install OpenSSL for Prisma compatibility, nginx for reverse proxy, and ffmpeg for video processing
+RUN apk add --no-cache openssl nginx ffmpeg
+
+# Install pnpm globally
+RUN npm install -g pnpm
+
+# Copy workspace configuration files
+COPY package.json pnpm-workspace.yaml pnpm-lock.yaml ./
+COPY packages/backend/package.json ./packages/backend/
+COPY packages/frontend/package.json ./packages/frontend/
+
+# Install ALL dependencies (including Prisma CLI needed for migrations)
+RUN pnpm install --frozen-lockfile
+
+# Copy backend build artifacts
+COPY --from=backend-builder /app/packages/backend/dist ./packages/backend/dist
+COPY --from=backend-builder /app/packages/backend/prisma ./packages/backend/prisma
+
+# Copy Prisma generated client (in pnpm workspace structure)
+# We need to copy the entire .pnpm directory to preserve the Prisma client
+COPY --from=backend-builder /app/node_modules/.pnpm ./node_modules/.pnpm
+
+# Copy frontend build artifacts
+COPY --from=frontend-builder /app/packages/frontend/dist ./packages/frontend/dist
+
+# Create uploads directory
+RUN mkdir -p /app/packages/backend/uploads
+
+# Copy nginx configuration
+COPY nginx.conf /etc/nginx/nginx.conf
+
+# Copy startup script
+COPY docker-entrypoint.sh /app/docker-entrypoint.sh
+RUN chmod +x /app/docker-entrypoint.sh
+
+# Expose frontend port
+EXPOSE 5173
+
+# Set entrypoint
+CMD ["/app/docker-entrypoint.sh"]
+