Hoist the GitHub stats into one shared constant

[GuanzhouSong]

Addresses the second half of #128 (the GitHub stats). The create_indexes_background row in the first half is content in documentdb/docs and needs a separate PR there — see the note at the bottom.

Problem

The star and fork counts were hardcoded independently in two files with no shared constant, so they drifted apart:

Metric	app/page.tsx	app/ai/page.tsx	Actual
Stars	3.2k+	3.4k+	3423
Forks	200+	240+	248

$ curl -s https://api.github.com/repos/documentdb/documentdb | jq "{stars: .stargazers_count, forks: .forks_count}"
{ "stars": 3423, "forks": 248 }

Both render in adjacent sections — "Built in the open" on / and "Built in the open, backed by a real ecosystem" on /ai — so a visitor moving between the two pages sees two different star counts for the same repository.

Worth being precise about severity: neither figure is false. The + suffix covers the actual numbers in both cases. The defect is that nothing stopped a refresh from updating one page and leaving the other behind, which is exactly what happened when #121 refreshed the AI page.

Fix

Both pages now read from app/services/projectStats.ts. The TSC member and organization counts are hoisted too — they are quoted on both pages and currently agree, so this keeps them from being the next thing to drift. They were verified against upstream MAINTAINERS.md: 11 members across Microsoft (4), Amazon (4), AB InBev (1), Rippling (1), YugabyteDB (1).

Values stay hardcoded rather than fetched at build time, so the build keeps no network dependency on a rate-limited unauthenticated API. The refresh command and the date last checked are recorded in the file so the next update is a one-line change in one place.

No rendered output changes on /ai; / picks up the newer figures.

Not in this PR

§1 of the issue — the create_index_background table row being wrong in name, schema, and description — lives in postgres-api/functions.md in documentdb/docs. articles/ is a gitignored build artifact here, cloned at build time per content.config.json, so the fix cannot land in this repo. I confirmed the upstream definition while validating: create_index_background--0.111-0.sql defines create_indexes_background (plural) under __API_SCHEMA_V2__, which Makefile:28 resolves to documentdb_api, not documentdb_api_internal. Happy to open that PR next.

npm is blocked on this machine, so lint and the build are left to CI. The change is a constant extraction with no logic.