GET /resorts feature and range filters (Issue #57)

[jonathanstelman]

Summary

• Extends GET /resorts with boolean feature flag filters and numeric range filters
• Adds 5 missing fields to ResortSummary needed by the new filters (has_night_skiing, has_terrain_parks, is_dog_friendly, has_snowshoeing, trail_length_mi)
• All params are optional and fully composable with each other and the existing filters from GET /resorts endpoint — basic #56

New query params

Param	Type	Behavior
has_alpine, has_cross_country, has_night_skiing, has_terrain_parks, is_dog_friendly, has_snowshoeing, is_allied	bool	Filter to resorts where field equals value
reservation_required	bool	true → reservation_status == "Required"
min_vertical / max_vertical	float	Inclusive vertical range (ft)
min_trails / max_trails	float	Inclusive trail count range
min_lifts / max_lifts	float	Inclusive lift count range
min_trail_length / max_trail_length	float	Inclusive trail length range (mi); resorts with null values are excluded

Test plan

• Each boolean flag filters correctly when true and false
• reservation_required=true returns only "Required" resorts
• reservation_required=false excludes "Required" resorts
• Range filters are inclusive at both bounds
• Resorts with null values for a range field are excluded
• Boolean + range + reservation filters compose correctly
• All 38 backend tests pass; pipeline tests unaffected; Black clean

Closes #57

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