docs: add /docs API reference for AI agents

[TheAlexPG]

Summary

• Adds a public /docs route on the frontend with a curated, agent-focused API reference covering auth, API keys, workspaces, objects, connections, diagrams, technologies, webhooks, realtime WebSocket, plus a roll-up of secondary endpoints (drafts, comments, activity, versions, members/invites, teams, packs, diagram access, flows, export, notifications).
• Mirrors the same content as Markdown under docs/api/ so it's also consumable directly from the repo (one file per section + an index).
• No backend changes. Reuses the dark LegalLayout aesthetic with a sticky table of contents on lg+ breakpoints, an Endpoint card primitive (color-coded methods), and a CodeBlock primitive.

Test plan

• npm run build in frontend/ passes (tsc -b && vite build — production build succeeds, no TypeScript errors).
• python -m pytest in backend/: the same 64 tests pass and 55 tests fail as on main. All failures are environment-only (Redis on localhost:6379 + Postgres not running in the local sandbox); they reproduce identically on main at HEAD before any of these changes. The repo's CI workflow does not run pytest, only Docker image builds — so this PR does not introduce any test regressions vs. main.
• Manual: visit /docs in dev to spot-check rendering and anchor links.

Notes for reviewers

• The page intentionally has zero runtime dependencies (no API calls, no React Query) so it stays available even when the backend is offline.
• The /docs route is public (no <ProtectedRoute> wrap) so AI agents can read the reference before authenticating, mirroring how /terms and /privacy are exposed.