feat: Implement user promo feed endpoint (/api/user/feed)

[RandomProgramm3r]

This commit introduces the GET /api/user/feed endpoint, enabling authenticated users to retrieve a personalized and paginated feed of promo.

Key features include:

• User-specific promo targeting based on their age and country attributes against promo targeting criteria (age_from, age_until, country).
• Dynamic calculation of a promo's 'active' status, considering:
  • Validity period (active_from, active_until) relative to current date (UTC).
  • Usage counts (used_count vs max_count) for 'COMMON' type promos.
  • Availability of unactivated codes for 'UNIQUE' type promos.
• Filtering capabilities via query parameters:
  • active (boolean): Filters promos based on their dynamic active status.
  • category (string): Allows case-insensitive filtering by promo category.
• Standard limit/offset pagination, including an X-Total-Count header that reflects the total number of promos after all filters are applied.
• Serialized promo data provides essential information (e.g., ID, company details, description, image URL) while intentionally omitting the actual promo code values to prevent premature activation.
• Promos in the feed are sorted by their creation date (created_at) in descending order by default.