feat(ci): build custom Docker images for Gitea Actions compatibility

- Create Dockerfile.ci-base with Bun, Node.js, and Playwright pre-installed
- Add build-ci-images workflow that triggers on package.json changes
- Update PR workflow to use custom ci-base image instead of GitHub Actions
- Update Release workflow to use custom ci-base image
- Remove dependency on oven/setup-bun@v2 GitHub Action
- Remove dependency on docker/setup-buildx-action GitHub Action
- Images are automatically built when dependencies in package.json update
- Weekly scheduled rebuild ensures tools stay current

This resolves the 'authentication required: Repository not found' error
when Gitea Actions tries to clone GitHub Actions from external repositories.

Dockerfile.ci-base

@@ -0,0 +1,32 @@
+# Base CI image with Bun, Node.js, Playwright, and build tools
+# Used for Gitea Actions CI workflows
+# Uses Microsoft Playwright image as base (Ubuntu-based) with Bun added
+
+FROM mcr.microsoft.com/playwright:v1.58.0-jammy AS base
+
+# Install unzip (required for Bun installation) and other tools
+RUN apt-get update && apt-get install -y unzip && rm -rf /var/lib/apt/lists/*
+
+# Install Bun (latest version)
+# Note: The playwright image already has Node.js pre-installed
+RUN curl -fsSL https://bun.sh/install | bash
+
+# Add Bun to PATH for subsequent commands
+ENV PATH="/root/.bun/bin:$PATH"
+
+# Verify installations
+RUN echo "=== Bun Version ===" && bun --version && \
+    echo "=== Node.js Version ===" && node --version && \
+    echo "=== Playwright Version ===" && bun x playwright --version
+
+WORKDIR /app
+
+# Set default environment variables
+ENV DATABASE_PROVIDER=sqlite
+ENV DATABASE_URL=file:./prisma/ci.db
+ENV BETTER_AUTH_SECRET=test-secret-key-for-ci-only
+ENV NODE_ENV=test
+
+# Health check command (can be overridden)
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+    CMD bun --version