[Backend] Structured Groq cover letter analysis with response caching

[Gbangbolaoluwagbemiga]

Context

Every analysis call hits Groq fresh with no caching — repeated requests for the same cover letter re-run inference.

What needs to be done

1. Structured output — return typed JSON:
   ```ts
   interface CoverLetterAnalysis {
   relevance_score: number; // 0-100
   communication_clarity: number; // 0-100
   timeline_feasibility: number; // 0-100
   red_flags: string[];
   summary: string; // ≤ 100 words
   }
   ```
   Use Groq's `response_format: { type: 'json_object' }`.

2. Cache in Supabase — SHA-256(cover_letter + job_id) → store result, expire after 24h

3. Graceful degradation — if Groq down, return partial response, not 500

Acceptance criteria

• Typed JSON response matching interface
• Cache hit on identical input (verifiable in logs)
• Supabase migration for `cover_letter_cache` table