Date-only strings cause 400 errors from STAC APIs

[AliceR]

Problem

STAC APIs return 400 errors when sending date strings like 2025-12-01. They require full datetime format like 2025-12-01T00:00:00Z.

Error message:

invalid datetime separator, expected `T`, `t`, `_` or space

Solution

Convert date-only strings to full ISO 8601 datetime format in makeDatetimePayload():

• 2025-12-01 → 2025-12-01T00:00:00Z (start of day)
• 2025-12-31 → 2025-12-31T23:59:59Z (end of day)

Keep existing datetime strings unchanged (if they already contain T).