docs(README): document per-account service overrides

[cristim]

Summary

Per-account service overrides have been a feature since migration 000011_cloud_accounts.up.sql:78 (mid-2024 era), and now have full UI coverage thanks to #72 (inline payment edit) and #106 (create modal). None of this is documented in README.md or any user-facing docs. A user landing on the dashboard for the first time sees the panel + the Add override modal but has no narrative for what overrides ARE, when they should be used, or how they interact with the global Settings → Purchasing defaults.

What to add

A short section in README.md (or whichever doc is the user entry-point) covering:

1. Concept: each account can override the global term/payment/coverage defaults for any (provider, service) pair. Overrides are stored per (account_id, provider, service) and the engine reads them when generating recommendations for that account.
2. When to use: examples — "this account is dev, prefer 1yr no-upfront for RDS"; "this production account always wants 3yr all-upfront for ElastiCache regardless of the global setting".
3. How to create: navigate to Settings → Accounts → expand an account row → click "Service overrides" → modal opens → pick service + override fields → Save. Cite the AWS-only V1 boundary (ux(settings): create-override modal for Azure and GCP accounts (V2 of #106) #109 tracks Azure/GCP).
4. How to edit: inline-edit Payment on existing rows (ux(settings): add payment option selector to AWS account overrides modal #72); Reset deletes the override; ux(settings): inline edit Term/Coverage + Enabled toggle on existing override rows (V2 of #72) #110 will widen this to Term/Coverage/Enabled.
5. What "Inherit" means: a field left blank inherits the global default as configured in Settings → Purchasing — sparse PUT semantics, not a stored "Inherit" value.
6. Migration considerations: existing scripted setups that use PUT /api/accounts/:id/service-overrides/... continue to work — the UI and the API target the same row.

Out of scope

• Backend-only knobs not surfaced in the UI (the include/exclude array fields). Document them once a UI for them lands.
• Migration steps for tenants that pre-existed before the override feature — there are none; the feature is additive on existing accounts.