GET /resorts blackout date and LTT filters (Issue #58)

[jonathanstelman]

Summary

• Adds blackout_dates and ltt_dates query params — comma-separated YYYY-MM-DD lists that exclude resorts blacked out on any of the given dates
• Adds ltt_available boolean filter for LTT program participation
• Adds ltt_available to ResortSummary response shape
• Refactors internal store from list[ResortSummary] → list[Resort] so filters can access full-model columns (blackout_all_dates, ltt_blackout_all_dates) without leaking those large fields into the API response; projection to ResortSummary now happens at return time

Filter behavior

Param	Logic
blackout_dates=2025-12-25,2025-12-26	Excludes resorts where any date appears in blackout_all_dates
ltt_dates=2025-12-25	Same logic against ltt_blackout_all_dates
ltt_available=true	Only resorts participating in the LTT program

Resorts with an empty blackout list always pass the date filters.

Test plan

• blackout_dates excludes resorts blacked out on any given date
• Multiple dates work (comma-separated)
• Date with no overlap returns all resorts
• Whitespace around dates is trimmed
• Resorts with empty blackout list always pass
• ltt_dates applies same logic against LTT-specific blackouts
• ltt_available=true/false filters correctly
• ltt_available present in response shape
• All three new params compose with each other and prior filters
• All 52 backend tests pass; pipeline tests unaffected; Black clean

Closes #58

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