refactor: remove legacy support from Docker and Makefile

Dockerfile changes:
- Replace pip with uv package manager for faster builds
- Remove build-essential and git dependencies
- Use pyproject.toml instead of requirements.txt
- Add uv installation and virtual environment setup
- Modernize environment variables and caching
- Fix hadolint warnings (add --no-install-recommends and pipefail)

Makefile changes:
- Remove docker-build-legacy target (legacy builder)
- Remove docker-dev-host and docker-prod-host targets (networking workarounds)
- Remove docker-reset target (networking fixes)
- Remove docker-clean-legacy target (duplicate functionality)
- Clean up help output and reduce maintenance overhead

This modernizes the build system and removes workarounds for older Docker versions and networking issues.

Dockerfile

@@ -1,21 +1,24 @@
 # UnitForge Docker Image
-# Simple build for development and production
+# Modern build for development and production
 
 FROM python:3.11-slim
 
 # Set environment variables
 ENV PYTHONUNBUFFERED=1 \
     PYTHONDONTWRITEBYTECODE=1 \
-    PIP_NO_CACHE_DIR=1 \
-    PIP_DISABLE_PIP_VERSION_CHECK=1
+    UV_CACHE_DIR=/tmp/uv-cache \
+    UV_PYTHON=python3
 
-# Install system dependencies
-RUN apt-get update && apt-get install -y \
+# Install system dependencies and uv
+SHELL ["/bin/bash", "-o", "pipefail", "-c"]
+RUN apt-get update && apt-get install -y --no-install-recommends \
     curl \
-    git \
-    build-essential \
+    && curl -LsSf https://astral.sh/uv/install.sh | sh \
     && rm -rf /var/lib/apt/lists/*
 
+# Add uv to PATH
+ENV PATH="/root/.cargo/bin:$PATH"
+
 # Create app user
 RUN groupadd --gid 1000 app && \
     useradd --uid 1000 --gid app --shell /bin/bash --create-home app
@@ -23,9 +26,12 @@ RUN groupadd --gid 1000 app && \
 # Set work directory
 WORKDIR /app
 
-# Copy dependency files and install Python dependencies
-COPY backend/requirements.txt backend/
-RUN pip install --no-cache-dir -r backend/requirements.txt
+# Copy dependency files
+COPY pyproject.toml uv.lock ./
+
+# Install dependencies with uv
+RUN uv venv && \
+    uv pip install -e ".[web]"
 
 # Copy source code
 COPY . .
@@ -38,6 +44,9 @@ RUN chown -R app:app /app
 
 USER app
 
+# Activate virtual environment
+ENV PATH="/app/.venv/bin:$PATH"
+
 EXPOSE 8000
 
 # Default command for development