Bug report: Subscription delivery time shifts by 1 hour after DST change

Bug Description

Please describe.

Subscription schedules are stored in UTC and don't adjust for Daylight Saving Time transitions. When DST changes, subscriptions fire 1 hour earlier or later than the user's intended local time.

Expected: Subscriptions should fire at the same wall-clock time year-round (e.g., always 8am CT, regardless of whether CT is currently CDT or CST).

Actual: A subscription which was set for 8am during CDT is now firing at 7am CST

E.g., A user sets a subscription for 8am CT during CDT (UTC-5). The start_date is stored as 13:00 UTC. After DST ends and CT shifts to CST (UTC-6), 13:00 UTC now equals 7am local time, and so the subscription arrives an hour early.

How to reproduce

Expected: Subscriptions should fire at the same wall-clock time year-round (e.g., always 8am CT, regardless of whether CT is currently CDT or CST).

Actual: A subscription which was set for 8am during CDT is now firing at 7am CST

E.g., A user sets a subscription for 8am CT during CDT (UTC-5). The start_date is stored as 13:00 UTC. After DST ends and CT shifts to CST (UTC-6), 13:00 UTC now equals 7am local time, and so the subscription arrives an hour early.

How to reproduce

1. Set project timezone to a region that observes DST (e.g., US/Central)
2. Create a subscription scheduled for a specific local time (e.g., 8am)
3. Wait for a DST transition to occur 📆 👴
4. Observe that the subscription now fires 1 hour off from the originally intended local time

Additional context

From: https://posthoghelp.zendesk.com/agent/tickets/43459 (moved to PostHog: https://us.posthog.com/project/2/support/tickets/44944)

Debug info

Session: https://us.posthog.com/project/sTMFPsFhdP1Ssg/replay/019ab87a-76a3-7b65-96ed-94c3b9c84109?t=0
Admin: http://go/adminOrgUS/018fcb66-e8df-0000-8bc3-a7b071baeea1 (project ID 71639)

No file or test is named. Start by tracing the subscription scheduling path and its UTC-to-local timezone handling, then reproduce the issue with a US/Central project across a DST transition. Done means a subscription set for a local wall-clock time continues firing at that time after the offset changes.