Fix year-boundary slot gap in normalise_dates

[jnioche]

Summary

Fixes #6 (the remaining missing 30-minute slot after the original workaround).

• Root cause: the old code clipped year-crossing chunks to Jan 1 00:00, which made the chunk's API URL end at Jan 1 00:01Z — still a cross-year request. The API silently truncates those at Dec 31 23:00, so the Dec 31 23:30 slot and (depending on end date) earlier slots were missed.
• Fix: split 30 min before year end (Dec 31 23:30) instead, keeping the URL within the current year. The next chunk then starts cleanly at Jan 1 00:00.
• Known limitation: the Dec 31 23:30 slot (whose to field is Jan 1 00:00) is permanently inaccessible from the API — confirmed by direct API testing. No request (cross-year or within-year) returns it.
• TLS: switched reqwest to rustls-tls (default-features = false) so unit tests compile without requiring libssl-dev.

Test plan

• normalise_dates_splitting — updated to expect Dec 31 23:30 as last boundary
• normalise_dates_skipping_year — updated to expect Dec 31 23:30 / Jan 1 split
• normalise_dates_year_boundary_slot — new test verifying boundary structure for any cross-year range
• normalise_dates_end_within_current_year — new regression test: ranges that stay within a year are unaffected
• All 17 tests pass (cargo test)

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