Centralize Renovate config for openedx repos (frontend + backend)

[feanil]

Problem

40+ Open edX repos maintain their own near-duplicate renovate.json, and they've drifted over time — same intent, slightly different shapes. There's no single place to update org-wide policy (automerge rules, schedules, grouping, vulnerability alerts). Additional repos likely have no Renovate config at all and should be brought into the same scheme.

Two distinct patterns exist today:

• Frontend (~27 repos): config:base, automerge @edx/@openedx minor+patch, rebaseStalePrs, often patch.automerge: true. Some add weekly schedules, eslint grouping, or semanticCommits.
• Backend (~13 repos): config:base, broader automerge (minor, patch, pin, digest). A handful add schedules, prConcurrentLimit, or reviewer teams.

Phase 1 — Discovery

Content moved to sub-task

Phase 2 — Implementation

• Create the host repo / presets per Phase 1 decisions.
• Migrate the POC repos (1–2 FE, 1–2 BE) and let them bake long enough to see at least one real Renovate cycle. Adjust presets based on what we learn.
• Roll out to the remaining repos. Likely tracked as per-repo follow-up issues filed off this epic.
• Document the presets, override pattern, and how to onboard a new repo.

Out of scope (for this epic)

• Changing org-wide automerge / scheduling policy. This epic preserves current behavior; policy changes go in follow-ups against the central repo once it exists.

References

• Renovate shared config presets: https://docs.renovatebot.com/config-presets/
• Existing FE example: openedx/frontend-platform/renovate.json
• Existing BE example: openedx/credentials/renovate.json