Performance: Add LRU cache for rendered PDF pages

[cbcoutinho]

Problem

Each PDF preview request re-downloads and re-renders the PDF, even for the same page. This is inefficient when users navigate back and forth between pages.

Recommendation

Add a simple LRU cache for rendered pages:

from functools import lru_cache
import hashlib

# Cache key: (user_id, file_path, page, scale, pdf_hash)
@lru_cache(maxsize=100)  # Cache last 100 rendered pages
def _render_pdf_page_cached(cache_key: tuple) -> tuple[bytes, int]:
    user_id, file_path, page_num, scale, pdf_hash = cache_key
    # Actual rendering logic here
    ...

# In get_pdf_preview():
pdf_hash = hashlib.sha256(pdf_bytes).hexdigest()[:16]
cache_key = (user_id, file_path, page_num, scale, pdf_hash)
png_bytes, total_pages = _render_pdf_page_cached(cache_key)

Benefit: Significantly reduces CPU load when users navigate between pages.

Trade-off: Memory usage (100 pages × ~500KB ≈ 50MB is reasonable).

Location

• nextcloud_mcp_server/api/visualization.py

Priority

Nice to have - Performance optimization

Parent Issue

Part of #502