Negative time values displayed in UI due to timezone mismatch

Bug Description

Conversations are displaying negative time values in the UI (e.g., "-850 seconds ago", "-949 seconds ago"). This occurs on the home page under "Recent Conversations" and potentially other pages displaying timestamps.

Environment

• Browser: Set to local European time (Germany)
• Server: May be running in non-UTC timezone (e.g., Europe/Berlin)

Root Cause

The backend uses datetime.now() (naive local time) in several places instead of datetime.now(timezone.utc) or the standard utc_now() helper. When the server runs in a non-UTC timezone:

1. Server writes local time (e.g., 2026-04-27T15:37:04 in German time) without timezone indicator
2. Frontend parseDateAsUTC() assumes naive timestamps are UTC, parsing it as 2026-04-27T15:37:04Z
3. The actual UTC time was 2026-04-27T13:37:04Z (2 hours earlier)
4. The timestamp appears in the future, causing now - timestamp to be negative

Affected Files

Enterprise code (enterprise/server/utils/conversation_callback_utils.py):

• Line 118: callback.updated_at = datetime.now()
• Line 145: conversation.last_updated_at = datetime.now()
• Line 232: conversation_work.updated_at = datetime.now().isoformat()

Core code (openhands/events/stream.py):

• Line 168: event._timestamp = datetime.now().isoformat()

Other files:

• enterprise/server/logger.py:42: obj = {"ts": datetime.now().isoformat(), **obj}

Suggested Fix

Replace all datetime.now() calls with timezone-aware UTC datetimes:

# Option 1: Use the existing utc_now() helper
from openhands.agent_server.utils import utc_now
callback.updated_at = utc_now()

# Option 2: Use datetime with timezone directly  
from datetime import datetime, timezone
callback.updated_at = datetime.now(timezone.utc)

Why _fix_timezone() Does Not Solve This

The _fix_timezone() method in sql_app_conversation_info_service.py adds UTC timezone to naive datetimes when reading from the database:

def _fix_timezone(self, value: datetime | None) -> datetime:
    if not value.tzinfo:
        value = value.replace(tzinfo=UTC)
    return value

This only works if the original datetime was actually in UTC. When datetime.now() writes local time, _fix_timezone() incorrectly labels it as UTC.

Impact

• Severity: Minor (cosmetic)
• User Impact: Confusion when viewing conversation timestamps
• Scope: Recent Conversations list, potentially other timestamp displays

Steps to Reproduce

1. Run OpenHands server in a non-UTC timezone (e.g., Europe/Berlin)
2. Create or update a conversation
3. View the "Recent Conversations" list on the home page
4. Observe negative time values for recently updated conversations

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This issue was created by an AI agent (OpenHands) based on a customer report.

Start with the listed datetime.now() calls in enterprise/server/utils/conversation_callback_utils.py, enterprise/server/logger.py, and openhands/events/stream.py, then inspect the existing utc_now() helper and _fix_timezone() behavior. Update the affected timestamp writes to use UTC-aware values and reproduce the issue in a non-UTC timezone to confirm that recent conversation times are no longer negative.