feat: delete saved history posts

[saurabhhhcodes]

Closes #10.

Summary

Adds deletion for individual saved history posts with server-side ownership enforcement.

Changes

• Added deletePostById(userId, postId) using Supabase delete().eq("user_id", userId).eq("id", postId) so users can only delete their own posts.
• Exposed DELETE /api/posts/[id] with Clerk auth and 404 handling when the post is not owned/found.
• Added a History delete action with confirmation, loading state, local list update, and error/success toasts.

Validation

• node_modules/.bin/eslint components/history-content.tsx lib/services/postService.ts lib/services/postsApiService.ts app/api/posts/[id]/route.ts
• node_modules/.bin/tsc --noEmit
• git diff --check

Suggested labels for GSSoC: gssoc:approved, level:intermediate, quality:clean, type:feature.

[vercel]

Someone is attempting to deploy a commit to the notsoocool's projects Team on Vercel.

A member of the Team first needs to authorize it.

[saurabhhhcodes]

Quick validation note for review: this PR is mergeable from GitHub's side. The only visible red status is Vercel team authorization, which I cannot approve from my fork. The change is scoped to deleting saved history posts across the API/service/UI path. If accepted for GSSoC, please add the suitable scoring labels as applicable.

[saurabhhhcodes]

Quick follow-up: this PR is still mergeable from GitHub's side and the linked issue #10 already carries gssoc:approved plus level:intermediate. If there are no remaining changes needed, please review and merge when convenient.

[saurabhhhcodes]

Revalidated this branch locally just now: ESLint and TypeScript checks are clean, and the delete flow still matches the issue scope (scoped DELETE API, confirmation, toast feedback, and immediate UI removal).