docs: README presentation polish + community-file gaps

Add a table of contents to the README, extend the comparison matrix with
WHOOP and Google Health/Fitbit columns plus rows for multi-source dedup,
provider aggregation, and encryption at rest, and add a CI status badge.
Reconcile two stale claims against the current code: the Prisma model
count (now 60) and the AI architecture note (add Codex). Strengthen the
Security and Privacy section with the bring-your-own-key / local-endpoint
AI posture.

Add .github/FUNDING.yml so the GitHub Sponsor button renders, and fix the
contributor prerequisite to Node.js 22 to match the Dockerfile base image.

(cherry picked from commit 7cfe326709814dd0ce16d1554d5afd5933de448c)

.github/FUNDING.yml

@@ -0,0 +1,2 @@
+# Funding links for the GitHub "Sponsor" button.
+custom: ["https://buymeacoffee.com/mbombeck"]