[Bug]: Vector store timestamp inconsistency — MemoryNode uses naive datetime while collection metadata uses UTC

## QwenPaw Version

`1.1.8.post1` (commit `68cd42d2`)

## Description

`MemoryNode.get_now_time()` in the `reme` package uses `datetime.datetime.now()` which returns a **naive datetime** (no timezone info) in the system's local time. However, collection/dataset-level metadata in the vector store uses UTC timestamps. This causes an 8-hour discrepancy in regions using UTC+8 (e.g., Asia/Shanghai).

The mismatch means:
- Individual memory node timestamps (`time_created`, `time_modified`) are in local time (e.g., `2026-05-26 14:30:00`)
- Collection-level metadata (creation time, update time) are in UTC (e.g., `2026-05-26 06:30:00`)
- Queries or displays that mix both levels show inconsistent times

## Component(s) Affected

- [x] Core / Backend (app, agents, config, providers, utils, local_models)
- [ ] Console (frontend web UI)
- [ ] Channels (DingTalk, Feishu, QQ, Discord, iMessage, etc.)
- [ ] Skills
- [ ] CLI
- [ ] Documentation (website)
- [ ] Tests
- [ ] CI/CD
- [ ] Scripts / Deploy

## Environment

- **QwenPaw version:** `1.1.8.post1`
- **OS:** Windows 11 (AMD64, timezone: Asia/Shanghai, UTC+8)
- **Install method:** from source
- **Python version:** 3.12

## Steps to Reproduce

1. Configure QwenPaw with timezone `Asia/Shanghai` (UTC+8)
2. Interact with the Agent to trigger memory storage (vector DB write)
3. Compare timestamps at the memory node level vs. collection/dataset level
4. Observe ~8 hour difference between the two

## Actual vs Expected

- **Actual:** `MemoryNode.time_created` uses local time (e.g., `2026-05-26 14:30:00`) while collection metadata uses UTC (e.g., `2026-05-26 06:30:00`)
- **Expected:** All timestamps should use the same timezone base, preferably timezone-aware datetimes or consistently UTC.

## Logs / Screenshots

**Affected code** (`reme/core/schema/memory_node.py:17-23`):

```python
def get_now_time() -> str:
"""Get current timestamp in YYYY-MM-DD HH:MM:SS format."""
return datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
```

`datetime.datetime.now()` returns a naive datetime in the system's local time. If QwenPaw is configured for `Asia/Shanghai`, but the vector store backend (e.g., ChromaDB) uses UTC for collection metadata, the two timestamps diverge by the UTC offset.

## Additional Notes

This affects users in non-UTC timezones. Users in UTC timezone (offset 0) would not observe this issue.

The fix would need to be in the `reme` package (dependency), not in QwenPaw core. Suggested approaches:
- Use `datetime.datetime.now(datetime.timezone.utc)` and store all timestamps in UTC
- Or make `get_now_time()` accept a timezone parameter

(These are suggestions for maintainers to evaluate, not prescribed solutions.)