🐛 fix(server): Dockerfile uses venv + copies source before install

Two stacked issues:
- python:3.12-slim marks system Python as externally-managed (PEP 668), so
  `uv pip install --system` fails. Switch to a project venv at /opt/venv
  and put it on PATH.
- setuptools needs the opds_sync/ source tree at install time, but the
  original COPY ordering installed before copying. Reorder.

Verified locally: image builds and `from opds_sync.main import create_app`
works in the resulting container.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vitofico

server/Dockerfile

@@ -2,19 +2,25 @@ FROM python:3.12-slim AS base
 
 ENV PYTHONUNBUFFERED=1 \
     PYTHONDONTWRITEBYTECODE=1 \
-    PIP_NO_CACHE_DIR=1
+    PIP_NO_CACHE_DIR=1 \
+    VIRTUAL_ENV=/opt/venv \
+    PATH=/opt/venv/bin:$PATH
 
 WORKDIR /app
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
         build-essential libpq-dev \
  && rm -rf /var/lib/apt/lists/*
 
-COPY pyproject.toml ./
-RUN pip install --no-cache-dir uv && \
-    uv pip install --system "."
+# uv installs into a project venv (avoids PEP 668 on python:3.12-slim, which
+# marks the system Python as externally-managed).
+RUN pip install --no-cache-dir uv && uv venv "$VIRTUAL_ENV"
 
+# Source needs to be present so setuptools can discover the package.
+COPY pyproject.toml ./
 COPY opds_sync ./opds_sync
+RUN uv pip install "."
+
 COPY migrations ./migrations
 COPY alembic.ini ./