bd-ko1 Part B: detail-page enrichment for min_contract_months

[MukhammadIbrokhimov]

Summary

• New enrich pipeline stage between scrape and match. Fetches detail pages for canonical WG-Gesucht / Kleinanzeigen flats missing min_contract_months, parses the field, and persists it so filter_contract (filters.py:120-128) actually fires.
• Adds flats.available_until (forward-migration column) and extends filter_short_term to use the structured available_from/available_until span before falling back to the bd-33h title/description heuristic.
• Caps per-pass detail fetches via --budget (default 30) and sleeps 2 s between requests. Fetch failures are logged and the flat row is left untouched so the cheap keyword filter remains the safety net.

Test plan

• pytest tests/test_matcher_filters.py tests/test_pipeline_enrich.py tests/test_wg_gesucht_scraper.py tests/test_kleinanzeigen_scraper.py -v — green
• pytest -q — full suite green (589 passed)
• ruff check on touched files — no errors
• Manual: flatpilot enrich --budget 3 against a real DB with at least one new WG-Gesucht flat; verify the row's min_contract_months is populated.

Closes FlatPilot-k1v.