feat: add vector viz template and chunk context endpoint

Extracted vector visualization HTML template to separate file to resolve
syntax conflicts between Jinja2, Alpine.js, and CSS. Added chunk context
endpoint for fetching matched chunks with surrounding text.

Changes:
- Moved vector_viz.html to templates/ directory (separates Jinja2/Alpine.js/CSS)
- Added /app/chunk-context endpoint for retrieving chunk text with context
- Updated .dockerignore to include HTML files in Docker builds
- Moved anthropic and boto3 to main dependencies (needed for production features)
- Added jinja2 dependency for template rendering

Fixes Jinja2 TemplateSyntaxError caused by CSS colons being parsed as
Jinja2 syntax when template was inline in Python code.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.dockerignore

@@ -5,3 +5,4 @@
 !uv.lock
 
 !nextcloud_mcp_server/**/*.py
+!nextcloud_mcp_server/**/*.html