docs: fix apps/ai references after consolidation, add Tantivy search docs

[devin-ai-integration]

Summary

Fixes incorrect documentation introduced in the parent PR #3847 and adds missing documentation for the new Tantivy-powered search.

Fixes to parent PR docs:

• developers/1.env.mdx: The parent PR incorrectly changed references from apps/api to apps/ai. Since refactor: consolidate apps/ai into apps/api #3852 consolidated apps/ai into apps/api (and apps/ai no longer exists), this restores the correct apps/api references. Also removes the deleted apps/api/package.json from the supabase usage note.
• developers/5.run.mdx: The parent PR changed the API run command to pnpm -F api dev and added a separate "AI Server (apps/ai)" section. Since apps/api is now a Rust project (no package.json), this restores cargo run -p api and removes the nonexistent apps/ai section.

New documentation:

• faq/1.features.mdx: Updates the search FAQ to mention Tantivy-powered full-text search (added in feat(search): implement search with tantivy #3831).
• faq/3.technical.mdx: Adds a new "How does in-app search work?" FAQ entry explaining the Tantivy search engine.

Stacked on #3847 (devin/1770768612-update-docs-recent-changes-2).

Review & Testing Checklist for Human

• Verify that apps/ai/ truly no longer exists on main and apps/api/Cargo.toml has name = "api" — confirming the cargo run -p api command and apps/api path references are correct
• Check whether the Tantivy FAQ description ("filtering by type... and date range, with results ranked by relevance") accurately reflects the current plugins/tantivy implementation
• Confirm no other recent code changes (last 7 days) need documentation that isn't covered by this PR or the parent docs: update technical FAQ, support FAQ, and AI server run instructions #3847

Notes

• This is a stacked PR — review only the incremental diff on top of devin/1770768612-update-docs-recent-changes-2
• Link to Devin run: https://app.devin.ai/sessions/cdd0ac51fdab4ceeb6238c74d5de38a5
• Requested by: unknown ()

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