fix: address critical security and performance findings from audit #001

[mrjunos]

Summary

Resolves all 5 critical findings from `audits/001_2026-04-06_audit_report.md`.

• CORS: replaced `allow_origins=["*"]` with env-configured allowlist (`COMPASS_ALLOWED_ORIGINS`). Defaults to `localhost:3000` and `localhost:5173`.
• Path traversal: sanitized upload filename with `Path(file.filename).name` — strips any directory components.
• Upload size limit: added 50MB cap, returns HTTP 413 if exceeded.
• SQLite index: added `CREATE INDEX IF NOT EXISTS idx_messages_session_id` — eliminates full table scan on every chat request.
• LLM timeout: added `timeout=60` to `litellm.acompletion` — prevents requests from hanging indefinitely if Ollama is unresponsive.

Also includes:

• Bug fix: `get_session_messages` was using `ORDER BY timestamp DESC` which is non-deterministic when messages share the same second. Changed to `ORDER BY id DESC` (AUTOINCREMENT, always unique).

Tests added (38 total, up from 26)

`test_api.py` — 3 new upload tests:

• Path traversal: `../../etc/passwd.md` → sanitized to `passwd.md` ✅ covered by `test_path_traversal_filename_is_sanitized`
• File too large: >50MB returns HTTP 413 ✅ covered by `test_413_when_file_too_large`
• File at exactly 50MB: accepted (boundary check) ✅ covered by `test_file_at_size_limit_is_accepted`

`test_database.py` — 7 new tests (new file):

• Schema: messages table and `session_id` index exist after `init_db`
• Save/get: session isolation, chronological order, limit behavior, empty session

Test plan

• 38 tests passing locally
• CI green
• Upload rejects files > 50MB — verified via `test_413_when_file_too_large`
• Path traversal sanitized — verified via `test_path_traversal_filename_is_sanitized`

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