Vendor htmx, add Dockerfile, show a placeholder for missing covers

Serve htmx 2.0.4 from the package instead of the jsdelivr CDN. Add a Dockerfile for local testing. Cover images now sit in a styled placeholder box that stays visible when Music Assistant has no cover for an album.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,21 @@
+FROM python:3.13-slim
+
+COPY --from=ghcr.io/astral-sh/uv:0.12 /uv /usr/local/bin/uv
+
+WORKDIR /app
+ENV UV_COMPILE_BYTECODE=1 UV_LINK_MODE=copy
+
+# Dependencies first, so code changes don't reinstall them.
+COPY pyproject.toml uv.lock ./
+RUN uv sync --locked --no-dev --no-install-project
+COPY src ./src
+RUN uv sync --locked --no-dev
+
+RUN useradd --system --no-create-home app && mkdir /data && chown app /data
+USER app
+
+ENV PATH=/app/.venv/bin:$PATH \
+    DB_PATH=/data/tag_albums.sqlite3
+VOLUME /data
+EXPOSE 8087
+CMD ["tag-albums", "serve", "--port", "8087"]