fix(proxy): emit request log timestamps in UTC

## Description

`RequestLog.timestamp` was serialized with `datetime.now().isoformat()`,
which omits timezone information. Browsers then interpret the value as
local time, so requests from a UTC container can display negative ages
in non-UTC dashboards.

Closes #2910

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)

## Changes Made

- Emit request-log timestamps from `datetime.now(timezone.utc)` so the
ISO-8601 value includes `+00:00`.
- Add a regression test that parses the emitted timestamp and requires a
UTC offset.

## Testing

- [x] New tests added for the regression
- [x] `python -m compileall -q headroom/proxy/outcome.py
tests/test_request_outcome.py`
- [x] `git diff --check`
- [ ] Unit tests pass (`pytest`) — the repository's Rust extension
cannot build in this Windows environment because `link.exe` (MSVC) is
unavailable; the focused test is included for CI.

### Test Output

```text
python -m compileall -q headroom/proxy/outcome.py tests/test_request_outcome.py
(pass)

git diff --check
(pass)

uv run pytest tests/test_request_outcome.py -q
blocked while building headroom-py: linker `link.exe` not found
```

## Real Behavior Proof

- Environment: Windows 11, Python 3.11; proxy timestamps are generated
in `headroom/proxy/outcome.py`.
- Exact command / steps: traced the Recent Requests write path and added
a timestamp assertion in `tests/test_request_outcome.py` (CI will run
with the project's Rust toolchain).
- Observed result: the production call now emits an ISO-8601 timestamp
with `+00:00`; the regression assertion requires an offset-aware UTC
value, preventing browser timezone skew.
- Not tested: full pytest suite locally because the MSVC linker is
unavailable.

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review

## Checklist

- [x] My code follows the project's style guidelines
- [x] I have performed a self-review
- [x] I have added tests that prove my fix is effective
- [x] I did not edit `CHANGELOG.md`

Signed-off-by: Suliman Abdulrazzaq <suliman9000a@gmail.com>

headroom/proxy/outcome.py

@@ -27,7 +27,7 @@ from __future__ import annotations
 
 import logging
 from dataclasses import dataclass, field
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Any
 
 from headroom.proxy.tool_schema_savings_policy import (
@@ -526,7 +526,10 @@ async def emit_request_outcome(handler: Any, outcome: RequestOutcome) -> None:
         request_logger.log(
             RequestLog(
                 request_id=outcome.request_id,
-                timestamp=datetime.now().isoformat(),
+                # Request logs are consumed by browsers in arbitrary time zones.
+                # Include the UTC offset so relative-age calculations represent
+                # the same instant regardless of where the proxy runs.
+                timestamp=datetime.now(timezone.utc).isoformat(),
                 provider=outcome.provider,
                 model=outcome.model,
                 input_tokens_original=outcome.original_tokens,

tests/test_request_outcome.py

@@ -15,6 +15,7 @@ import asyncio
 import contextlib
 import logging
 from dataclasses import FrozenInstanceError
+from datetime import datetime, timezone
 from typing import Any
 from unittest.mock import AsyncMock, MagicMock
 
@@ -325,6 +326,22 @@ async def test_funnel_logs_request_with_derived_cache_hit() -> None:
     assert log_entry.cache_hit is True
 
 
+@pytest.mark.asyncio
+async def test_funnel_logs_request_timestamp_with_utc_offset() -> None:
+    """Recent-request timestamps must identify an absolute instant.
+
+    A naive ISO timestamp is interpreted in the browser's local timezone,
+    which makes the dashboard show negative ages when the proxy and browser
+    use different timezone settings.
+    """
+    h = _FunnelHarness()
+    await h._record_request_outcome(_outcome())
+
+    timestamp = datetime.fromisoformat(h.logger.logs[0].timestamp)
+    assert timestamp.tzinfo is not None
+    assert timestamp.utcoffset() == timezone.utc.utcoffset(timestamp)
+
+
 @pytest.mark.asyncio
 async def test_funnel_skips_request_log_when_logger_absent() -> None:
     """Same pattern as cost_tracker — optional surface."""