docs(server): restore Better Auth integration guide for v2 - #1992

✅ No new issues found.

Reviewed changes — a single new doc page apps/content/docs/integrations/better-auth.mdx that restores the Better Auth integration guide for v2. It covers two patterns: resolving the session in a requireSession middleware (headers from initial context, getSession lookup, ORPCError('UNAUTHORIZED') on absence), and reusing a session already resolved outside the oRPC handler (typed as Awaited<ReturnType<typeof auth.api.getSession>>, narrowed by the middleware while public handlers check for it). Both show public plus protected procedures, inferred session types, handler setup with RPCHandler, and the cookie-header-forwarding caveat.

I verified the load-bearing claims against the repo and Better Auth's docs/source:

• The oRPC v2 API usage (os.$context, base.middleware, base.use, next({ context }) merge, RPCHandler.handle with prefix/context) matches the existing context.mdx, middleware.mdx, and fetch-api.mdx docs.
• auth.api.getSession({ headers }) matches Better Auth's official API, and the returned type { session, user } | null (confirmed from the better-auth get-session route source) lines up with the Session typing and both hander narrowings in the examples.
• Both external links resolve and their anchors exist.
• The page is auto-discovered by Blume's filesystem content source and its sidebar.label matches sibling integration pages, so no navigation config change is needed.

Technical details below only for the note about the PR description, which is editorial and non-blocking.

ℹ️ PR description says "Draft" while the PR is ready

The description still ends with "Draft while the proposed scope is being discussed.", but the PR is currently marked ready for review (draft: false). Consider updating the description to reflect the current state, e.g. noting the scope decision (docs-first, dedicated package still open per #1892).

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