[FEATURE] Add Advanced Filtering & Sorting

[JoshuaAFerguson]

Objective

Implement advanced filtering and sorting for list endpoints.

Query Syntax

GET /sessions?filter=status:running,template:ubuntu&sort=-created_at
GET /sessions?filter=user.email:contains:@example.com

Operators

• :eq: - equals
• :ne: - not equals
• :gt:, :lt: - greater/less than
• :contains: - string contains
• :in: - in list

Acceptance Criteria

• Filtering on all major fields
• Multiple sort fields
• Query validation
• Documentation with examples

Files

• api/internal/query/filter.go (NEW)
• api/internal/query/sort.go (NEW)