fix(accounts): editing one service-override field clears the override's other fields #1775

Problem

Editing one field of an account service override clears the override's other fields.

• Frontend. The inline editors in Settings send ONE field per PUT /api/accounts/{id}/service-overrides/{provider}/{service}: payment only (frontend/src/settings.ts :828), term only (:953), coverage only (:1046), enabled only (:1123).
• Backend. buildServiceOverride (internal/api/handler_accounts.go ~:1218) builds a FRESH row. existing supplies only ID and CreatedAt, and only the request's fields are applied.
• Store. The upsert (internal/config/store_postgres_config_audit.go ~:172) then overwrites every column: enabled, term, payment, coverage, ramp_schedule and all include/exclude filters.

So changing an account's Term override sets its payment, coverage and enabled to NULL (inherit) and clears every include/exclude filter. The response is 200 and the toast says it succeeded.

Why it matters

The overrides scope what gets purchased for that account. A cleared exclude_regions / exclude_types / exclude_engines widens what can be bought, with no warning.

Fix

Merge the request over the existing row, presence-aware: a key that is absent keeps the stored value, an explicit null clears it to inherit, and a value replaces it. That's the same pattern as mergeServiceConfig (internal/api/handler_config.go ~:351) for PUT /api/config/service/.... The request struct is already all pointers with omitempty, and applyServiceOverrideFields is documented as "copies sparse request fields".

Tests:

• a single-field PUT over a fully populated override keeps every other field;
• a null clears just that field;
• the commitment-option combo check runs on the merged result.

Found by the #19 adapter audit.