GET /resorts Peak Rankings filters (Issue #59)

[jonathanstelman]

Summary

• Adds has_peak_rankings boolean filter (based on whether pr_total is non-null)
• Adds pr_total_min/pr_total_max and per-category min/max range params for all 10 PR score dimensions
• Adds pr_lodging, pr_apres_ski, pr_access_road, pr_ability_low, pr_ability_high multi-value categorical filters (same repeated-param pattern as region)
• Adds pr_total and pr_overall_rank to ResortSummary response projection

Design decisions

• All PR range filters exclude resorts with null PR data (consistent with numeric range behavior from GET /resorts endpoint — feature filters #57)
• Categorical filters are case-insensitive
• has_peak_rankings explicitly exposes the null check rather than relying on pr_total_min=1 as a proxy

Test plan

• has_peak_rankings=true/false filters correctly
• pr_total_min/max range works; null values excluded
• Per-category range params work independently
• Categorical params work with single and multiple values
• Categorical filter excludes resorts with no PR data
• All params compose with each other and with prior filters
• pr_total and pr_overall_rank present in response
• All 74 backend tests pass; pipeline tests unaffected; Black clean

Closes #59

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