docs: build workspace packages and require a replica set in quickstart

[romiluz13]

Refs #83, #84

Changes

• README Quickstart and apps/docs/quickstart.mdx now run bun run build after bun install. Workspace packages export ./dist/*, and apps/api dev does not build them (Quickstart: 'bun run dev' in apps/api fails on a fresh clone (workspace packages not built) #83).
• Both list the MongoDB topology requirement: a replica set or sharded cluster. Memongo writes use transactions, which standalone mongod does not support. Atlas and the repo's Docker stack (single-node replica set) qualify. The docs link the standalone conversion tutorial (Docs: replica set is required for writes; standalone mongod gives opaque ADD_FAILED #84).
• The quickstart runtime versions now match package.json (previously "Node.js 20+ / Bun 1.2+"). A short note explains that /health is liveness only and identifies the standalone ADD_FAILED cause.

Clean-environment verification

A fresh clone of this branch, with an empty Bun install cache and no existing node_modules, dist, or Turbo cache, on Bun 1.4.2 and macOS:

Step	Result
bun install --frozen-lockfile	exit 0
cd apps/api && bun run dev before building	ERR_MODULE_NOT_FOUND for @memongo/lib/dist/index.js (reproduces #83)
bun run build	exit 0, 11/11 tasks, 0 cached
cd apps/api && bun run dev after building	listening; /health 200 on Node 26.10.0 and 22.23.2
Documented walkthrough against an existing Atlas cluster (replica set), isolated throwaway database, no provider keys	/health, /v1/status, /v1/add, /v1/search all 200; search returned exactly one result containing the README sentence, with no degradation
node scripts/check-docs-integrity.mjs	passed (14 pages)

An initial connection attempt timed out for an undetermined reason. The later attempt used a longer connection allowance, and the startup wait was extended after observing index creation continue beyond 60 seconds; no Memongo runtime code changed. The throwaway database drop timed out in normal cleanup; a separate cleanup run then removed it and confirmed it was gone.

Not verified: the local Docker path, a standalone mongod (the ADD_FAILED symptom comes from #84), Node 20.19 (the minimum), and other installation variants.

Out of scope: #84 suggestions 2 and 3 (fail fast on standalone at startup and include the cause in ADD_FAILED) change runtime behavior and are left for a separate change. Another option for #83 is a Turbo dev dependency on ^build; this PR does not make that change.

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