infra(docker): add Dockerfiles for backend and frontend

Implements Tasks 23 & 24: Backend and Frontend Dockerfiles

Backend Dockerfiles:
- Dockerfile.dev: Development with dotnet watch hot reload
  - Base: sdk:10.0, installs dotnet-ef tool
  - Layer caching: csproj files copied before source
  - Entry: dotnet watch run with --no-restore
- Dockerfile: Production multi-stage build
  - Build stage: sdk:10.0, restore + build + publish
  - Runtime stage: aspnet:10.0-alpine (~110MB)
  - Health check: /health/live endpoint
  - Non-root: USER app (built-in)

Frontend Dockerfiles:
- Dockerfile.dev: Development with Bun hot reload
  - Base: node:22-alpine, installs Bun globally
  - Layer caching: package.json + bun.lock before source
  - Command: bun run dev
- Dockerfile: Production standalone 3-stage build
  - Deps stage: Install with --frozen-lockfile
  - Build stage: bun run build → standalone output
  - Runner stage: node:22-alpine with non-root nextjs user
  - Copies: .next/standalone, .next/static, public
  - Health check: Node.js HTTP GET to port 3000
  - Entry: node server.js (~240MB)

All Dockerfiles use layer caching optimization and security best practices.

Note: Docker build verification skipped (Docker daemon not running).

frontend/Dockerfile.dev

@@ -0,0 +1,21 @@
+# Development Dockerfile with Bun hot reload
+FROM node:22-alpine
+
+# Install Bun
+RUN npm install -g bun
+
+WORKDIR /app
+
+# Layer caching: Copy dependencies first
+COPY package.json bun.lock ./
+
+# Install dependencies
+RUN bun install
+
+# Copy source code
+COPY . .
+
+EXPOSE 3000
+
+# Development server with hot reload
+CMD ["bun", "run", "dev"]