feat: add api/templates.json endpoint

[nemanjafleek]

Why?

Expose templates.json via api/templates.json public endpoint so the Dashboard can consume it.

How?

• Add static Astro endpoint.

Tickets?

• PLAT-1789
• https://discord.com/channels/1045027913260617789/1333770266668437534

Contribution checklist?

• The commit messages are detailed
• The build command runs locally
• Assets or static content are linked and stored in the project
• Document filename is named after the slug
• You've reviewed spelling using a grammar checker
• For documentation, guides or references, you've tested the commands and steps
• You've done enough research before writing

Security checklist?

• Sensitive data has been identified and is being protected properly
• Injection has been prevented (parameterized queries, no eval or system calls)
• The Components are escaping output (to prevent XSS)

References?

Optionally, provide references such as links

Preview?

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Latest commit: 91d185b

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