fix: detect negative UTC offsets (-HH:MM) in ISO timestamp parsing

[voidborne-d]

Bug

Three functions in trendradar/utils/time.py — format_iso_time_friendly, is_within_days, and calculate_days_old — detected timezone-aware timestamps using:

if "+" in iso_time or iso_time.endswith("Z"):

This correctly handles UTC (+00:00), positive offsets (+08:00, +05:30) and Z-suffix timestamps, but silently misses RFC 3339 negative UTC offsets (-05:00 EST, -08:00 PST, -03:30 Newfoundland, etc.).

When a negative-offset timestamp like 2025-01-01T00:00:00-05:00 is encountered, the check evaluates to False and the code falls through to the bare fromisoformat() path, which strips timezone info and calls pytz.UTC.localize() — reinterpreting the local time as UTC.

Impact:

• format_iso_time_friendly: displayed times up to 12 hours wrong for RSS articles from US/European sources (Hacker News, Reddit, NYT, BBC…)
• is_within_days: freshness filtering incorrect — articles from western-timezone sources could be wrongly kept or wrongly filtered
• calculate_days_old: day-count calculation silently wrong

Fix

Extract _has_tz_info(iso_time) helper that correctly checks for Z, +HH:MM, and -HH:MM:

def _has_tz_info(iso_time: str) -> bool:
    if iso_time.endswith("Z"):
        return True
    if len(iso_time) >= 6:
        suffix = iso_time[-6:]
        if suffix[0] in ("+", "-") and suffix[3] == ":":
            return True
    return False

Replace the old "+ in" check with _has_tz_info() in all three functions.

Changes

• trendradar/utils/time.py: add _has_tz_info() helper; apply it in format_iso_time_friendly, is_within_days, calculate_days_old
• tests/__init__.py: new file (test package)
• tests/test_time_utils.py: 41 new tests covering all three functions and the regression

Tests

41 tests, all passing:

Class	Count	Key tests
TestHasTzInfo	13	parametrized detection, regression test
TestFormatIsoTimeFriendly	11	EST/PST/IST/Newfoundland conversions, cross-TZ agreement
TestIsWithinDays	12	freshness filter with negative offsets, boundary values
TestCalculateDaysOld	5	negative offset regression, cross-TZ agreement