log: silence 'service config not found' on per-rec lookup (closes #263)

[cristim]

Summary

• PostgresStore.GetServiceConfig was returning a plain error on pgx.ErrNoRows, not wrapping ErrNotFound, so globalConfigCache.lookup propagated it as a real error on every recommendation refresh for services without a global config row.
• Wrap the no-rows path as %w ErrNotFound in both PostgresStore.GetServiceConfig (production) and the testablePostgresStore mock so callers get a consistent sentinel.
• In globalConfigCache.lookup, use errors.Is(err, ErrNotFound) to treat the absent-config case as (nil, nil) rather than returning an error, silencing the INFO log noise on each refresh cycle.

Test plan

• go build ./... clean
• go test github.com/LeanerCloud/CUDly/internal/api/... github.com/LeanerCloud/CUDly/internal/config/... passes (1906 tests)
• TestGetServiceConfig_NotFound still asserts assert.Error + assert.Contains("not found") -- both hold because the wrapped error message still contains "not found"
• TestResolveAccountConfigsForRecs_GlobalAbsentCachedNegative confirms the absent-global case is cached as missing (no second fetch) with no error returned

Closes #263.

[cristim]

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ℹ️ Review info

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Run ID: cfb9e71f-aafb-4059-9f6c-01f519c9ee45

📥 Commits

Reviewing files that changed from the base of the PR and between 4956d66 and fbf8092.

📒 Files selected for processing (3)

• internal/config/recommendation_overrides.go
• internal/config/store_postgres.go
• internal/config/store_postgres_mock_test.go

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• Create PR with unit tests
• Commit unit tests in branch fix/263-wave18

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