Expand cityless search, filters, and seller tools

[carlosorch]

Summary

Expand Wallapop MCP search and seller support while making global, cityless search the default.

Changes

• Default searches omit coordinates; explicit Spanish cities remain accent/case-insensitive.
• Resolve known cities locally and unknown Spanish cities through Nominatim.
• Return explicit errors instead of silently falling back to Valencia.
• Add native price, condition, shipping, age, and distance filters.
• Add bounded, deduplicated automatic pagination (max_pages, capped at 10).
• Enrich search results with IDs, descriptions, images, timestamps, seller/category IDs, condition, warranty, and refurbished status.
• Add seller listings and reviews tools, including deduplicated seller pagination.
• Fetch item details by public API when given an item ID, retaining SSR fallback for slugs and URLs.
• Update README, MCP guidance, and regression checks.

No authenticated Wallapop features or new dependencies were added.

Verification

uv lock --check
uv run python tests/test_locations.py
uv run python tests/test_features.py
uv run python -m compileall -q server.py tests
git diff --check

Live smoke checks covered global search, filtered pagination, item details through API and SSR, seller listings, and seller reviews.