Add CHANGELOG.md tracking template releases

[Copilot]

Description

Adds CHANGELOG.md to document template changes between tagged releases. Users can now track breaking changes, bug fixes, and new features across versions to decide when to upgrade their projects.

Covers changes from wasp-v0.18-template to wasp-v0.19-template:

• 61 commits analyzed across 23 merged PRs
• Categorized into: Added, Changed, Fixed, and Developer Experience
• Includes PR references and contributor attribution
• Follows Keep a Changelog format

Notable entries: Stripe payment improvements (#550, #505), Wasp v0.18.0 migration (#500), toast/file upload refactors (#472), asset optimizations (96% banner size reduction via SVG conversion #521).

Contributor Checklist

• Update e2e tests: If you changed the /template/app, then make sure to do any neccessary updates to /template/e2e-tests also.
• Update demo app: If you changed the /template/app, then make sure to do any neccessary updates to /opensaas-sh/app_diff also. Check /opensaas-sh/README.md for details.
• Update docs: If needed, update the /opensaas-sh/blog/src/content/docs.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>add changelog file</issue_title>
<issue_description></issue_description>

<agent_instructions>Create a changelog starting with the tag wasp-v0.18-template until wasp-v0.19-template tag. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

@Martinsos Hm what would be the process around it? We could, for every "release" we do, add changes from the last one to the Changelog. One option is also to stick to the process where each PR has to add info about itself to CHangelog. Although, you cuold say people can then just go and check the list of PRs on Github hm. @Martinsos Some extra motivation for this: https://discord.com/channels/686873244791210014/920312576133443634/1328281113277497354 .

What I get from this exchange is that it would be valuable if we listed bigger changes we did in each iteration of OpenSaas, e.g. in the ChangeLog.md, so people can see if there was some bug fix for example -> then they can react to that and also do the fix on their side. These would also be part of each release note. Kind of like we do for wasp.</comment_new>

• Fixes add changelog file #331

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