fix: 3-minute timeout on SSE LLM streams

From the PR description

Summary

• Wraps runLLMStream in both POST /chat and POST /projects/:projectId/chat with a Promise.race against a 180-second timeout
• On timeout, writes { type: "error", message: "Request timed out" } SSE event followed by [DONE] and closes the connection
• Prevents stalled upstream LLM API calls from holding SSE connections open indefinitely under load

Closes #100 Closes #114 Closes #117 Closes #119

Changes

• backend/src/routes/chat.ts - STREAM_TIMEOUT_MS = 180_000, Promise.race wrapping runLLMStream, timeout-specific error message in catch
• backend/src/routes/projectChat.ts - same pattern; STREAM_TIMEOUT_MS const placed after all imports
• backend/src/lib/__tests__/sseTimeout.test.ts - static analysis tests verifying both routes use Promise.race and the timeout
• backend/tsconfig.json - exclude __tests__ from tsc build
• backend/vitest.config.ts - include filter scoping tests to src/ to exclude compiled dist/ artifacts
• backend/package.json - "test": "vitest run" script added

Test plan

• Unit tests added and passing (3/3)
• Backend build passes