feat: improve RAG context budgeting (bounty #7)

[jing11223344]

Summary

Improves RAG context budgeting as described in issue #7 (Algora bounty clq18zr98000ejs0gt0nv7gwu).

Changes

New file: ui/utils/app/rag-context.ts
Shared RAG context preparation helper:

• parseBoundedInt — safe integer parsing with range clamping (env override support)
• formatSource — consistent source string formatting
• prepareRagContext — deduplication by content fingerprint, configurable character budget (default 8K), returns context + metadata
• defaultNResults — Chroma query count with bounds (default 8, max 20)

Updated: ui/pages/api/fetch-documents.ts

• Accept optional nResults parameter (default 8, cap 20)
• Return _prepared context alongside raw Chroma results
• Use CHROMA_PATH env var instead of hardcoded chroma-server:8000

Updated: ui/pages/api/rag-chat.ts

• Build fetch URL from request origin instead of hardcoded http://localhost:3000 (fixes hosted/Docker/proxy deployments)
• Consume _prepared context from fetch-documents
• Graceful fallback when document fetch fails

New file: ui/__tests__/utils/app/rag-context.test.ts
25 comprehensive tests covering:

• parseBoundedInt edge cases (null, NaN, min, max, float, string)
• formatSource with/without page number
• prepareRagContext dedup, budget truncation, empty/null docs, blank content, env override
• defaultNResults bounds checking

Testing

All 25 tests pass:

✓ __tests__/utils/app/rag-context.test.ts (25 tests)

Closes #7