[scheduler] Timezone Issue 7: Add timezone field to the event model

[rita-codes]

Current behavior:

• We accept TemporalSupportedObject as start/end values
• Those objects may not contain timezone info (e.g. 2025-03-05T12:00:00)
• When DST or cross-timezone rendering happens, we cannot know the original timezone

Solution:

• Add an explicit timezone field to SchedulerEvent
• This gives the engine a reliable data timezone

Benefits:

Date value	Has TZ info?	Event timezone provided?	Result
"2025-03-14T02:00:00Z"	✔️	optional	Treated as UTC → converted to the event timezone
"2025-03-14T02:00:00-05:00"	✔️	optional	Treated as UTC-5 → normalized to the event timezone
"2025-03-14T02:00:00"	❌	✔️	Interpreted as 02:00 in the event’s timezone
"2025-03-14T02:00:00"	❌	❌	Interpret as 02:00 in the adapter’s default timezone