Add referral system and user balance tracking

Introduce balance field to User model with Alembic migration. Add
referral menu, admin /refpay command, and deep link sharing. Update
Dockerfile to multi-stage build and add Postgres service to compose.
Fix deep link parsing logic and conditional proxy initialization.

Dockerfile

@@ -1,24 +1,38 @@
-FROM python:3.13-slim
+# Stage 1: Builder - Install dependencies
+FROM python:3.13-slim AS builder
+
+WORKDIR /build
+
+# Install build-time dependencies required for compiling packages like asyncpg
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    gcc \
+    libpq-dev \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY requirements.txt .
+RUN pip install --no-cache-dir --prefix=/install -r requirements.txt
+
+# Stage 2: Runner - Final lightweight image
+FROM python:3.13-slim AS runner
+
+ENV PYTHONUNBUFFERED=1 \
+    PYTHONDONTWRITEBYTECODE=1
 
 WORKDIR /app
 
-# Install system dependencies
+# Install runtime dependencies required by asyncpg
 RUN apt-get update && apt-get install -y --no-install-recommends \
-    build-essential \
+    libpq5 \
     && rm -rf /var/lib/apt/lists/*
 
-# Copy requirements first for better layer caching
-COPY requirements.txt .
+# Copy installed Python packages from the builder stage
+COPY --from=builder /install /usr/local
 
-# Install Python dependencies
-RUN pip install --no-cache-dir --upgrade pip && \
-    pip install --no-cache-dir -r requirements.txt
-
-# Copy entrypoint script
-COPY entrypoints/startup.sh /entrypoint.sh
-RUN chmod +x /entrypoint.sh
-
-# Copy application code (will be overridden by volume mount in development)
+# Copy application source code
 COPY . .
 
-ENTRYPOINT ["/entrypoint.sh"]
+# Ensure the startup script is executable
+RUN chmod +x entrypoints/startup.sh
+
+# Default command runs linting, migrations, and starts the bot
+CMD ["entrypoints/startup.sh"]