Add the Rescheduled event template (event-rescheduled) #272

Part of #5. For "new date" posts when an event, meetup or webinar moves.

Idea
A "Rescheduled" pill in the accent color sits at the top, with the event name large under it ("Frontend Meetup #14"). Under the name, the old date is shown struck through and dimmed ("Thu, 6 Nov, 7 PM"), with the new date next to it in bold, in the accent color ("Thu, 13 Nov, 7 PM"). An optional note goes at the bottom ("Same venue, your tickets still work").

Template details

• File: src/templates/event/rescheduled.ts, id event-rescheduled
• Fields: commonFields.title (the event name), oldDate (text, optional, default "Thu, 6 Nov, 7 PM"), newDate (text, default "Thu, 13 Nov, 7 PM"), note (text, optional, default "Same venue, your tickets still work"), commonFields.bgColor, commonFields.accentColor, commonFields.textColor

Where to look

• Closest reference: src/templates/event/webinar.ts (a badge at the top), src/templates/event/meetup.ts (event name and date), src/templates/ecommerce/price-drop.ts (an old value struck through next to the new one)

Acceptance criteria

• The old date is struck through and dimmed, and an empty oldDate or note (or only spaces) hides cleanly
• Long event names shrink with autoFontSize() and then truncate, and each date stays on one line
• The pill text stays readable on any accent color (contrastText()), and accent text stays readable on a light background
• Re-exported in src/templates/event/index.ts and registered in the Event block of src/templates/registry.ts
• Meets the shared template checklist in Add new card-style OG image templates #5

One template per contributor at a time, so more people get a chance.

Want to work on this? Comment below and I'll assign it to you. Please take one issue at a time, branch off dev, and open your PR against dev with Closes #272 in the description.