feat: add first-class background billing

[pjb157]

Summary

• add background as a first-class billing tier across credit transactions, batch aggregates, GenAI metrics, transaction responses, model pricing, and historical backfills
• resolve pricing as explicit background tariff → 24h batch tariff only; requests remain identified as background throughout, and stay unbilled when neither price exists
• expose background tariff configuration and effective pricing in the platform model dashboard independently of foreground SLA windows
• allow authorized connection syncs, including S3, to submit native background batches while preserving model validation and bypassing foreground SLA capacity and unverified-volume admission
• add an atomic, low-cost constraint migration that avoids scanning the large credits ledger

Why

Background dispatch is a distinct inference class, but billing and connector ingestion still treated it as an ordinary SLA batch in several paths. That could report the wrong tier, select realtime pricing, or reject/fail connector-created background work through foreground admission and completion-window parsing.

Impact

Platform managers can configure a dedicated background tariff per model. Without one, background work charges the model's 24h batch price without changing its accounting identity. Connection sync callers need the BackgroundInferenceUser role to select completion_window: "background"; accepted files become real background batches and do not consume foreground admission capacity.

Historical backfill scripts now classify file-backed and batchless background usage, including purged request rows recovered from analytics, without adding an extra analytics-table scan.

Validation

• cargo test -p dwctl background_ --lib (31 passed)
• cargo test -p dwctl test_compute_billing_tier --lib
• cargo test -p dwctl test_billing_tier_label_maps_each_case --lib
• just lint rust -- -D warnings
• just lint ts
• pnpm build
• focused dashboard tests (75 passed, including the previously flaky user-management test in isolation)
• live PostgreSQL migration and backfill SQL syntax checks
• independent code review with no remaining findings

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[Copilot]

Pull request overview

This PR makes “background” a first-class billing tier end-to-end (ledger, batch aggregates, pricing/tariffs, connection sync ingestion, metrics, and UI), with explicit pricing resolution rules that keep background requests labeled as background while preventing accidental realtime billing.

Changes:

• Add background-aware service tier classification/backfills for credits ledger and batch aggregates.
• Implement background-specific tariff resolution (explicit background tariff → 24h batch tariff only; otherwise unbilled) and expose it in APIs/UI.
• Allow connection sync to submit native background batches (role-gated) while bypassing foreground SLA capacity + unverified-volume admission but still validating models.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
scripts/backfill_credits_denorm.sh	Extends ledger backfill classification to include background (file-backed + batchless) and updates Phase 2 to recover background/flex from analytics.
scripts/backfill_batch_aggregates_denorm.sh	Backfills batch_aggregates.service_tier to include background based on fusillade batch metadata.
dwctl/src/request_logging/batcher.rs	Adds background billing tier computation, factors tariff resolution into a helper, and adds integration tests for background pricing/tier behavior.
dwctl/src/metrics/gen_ai.rs	Updates GenAI metrics tier labeling tests to include background.
dwctl/src/db/models/tariffs.rs	Documents background as a supported batch completion window in tariff models.
dwctl/src/db/models/credits.rs	Updates ledger response docs to include background service tier.
dwctl/src/db/handlers/tariffs.rs	Implements background pricing fallback semantics (background → 24h only) and adds tests.
dwctl/src/connections/sync.rs	Supports activating background batches from connection sync while skipping foreground admission/capacity reservation.
dwctl/src/api/models/transactions.rs	Updates API docs to include background service tier in transaction responses.
dwctl/src/api/models/tariffs.rs	Updates API docs to include background completion window for batch tariffs.
dwctl/src/api/models/deployments/mod.rs	Keeps background tariff visibility independent of foreground SLA windows, with 24h fallback visibility rules.
dwctl/src/api/models/deployments/enrichment.rs	Adds tests asserting background tariff and fallback aren’t filtered by foreground completion window config.
dwctl/src/api/models/connections.rs	Documents completion_window: background for sync trigger requests (role-gated).
dwctl/src/api/handlers/connections.rs	Enforces BackgroundInferenceUser role for background sync triggers and persists the completion window override.
dwctl/migrations/125_add_background_billing_tier.sql	Adds background to service tier CHECK constraints using a NOT VALID approach to avoid scanning the large ledger.
dashboard/src/utils/formatters.ts	Adds “Background” labeling and deterministic sort order for background tariffs.
dashboard/src/utils/formatters.test.ts	Tests background display name and sorting behavior.
dashboard/src/components/features/models/manage/ModelTariffTable.tsx	Allows selecting an explicit background tariff window even when foreground SLA windows are constrained.
dashboard/src/components/features/models/manage/ModelTariffTable.test.tsx	Verifies background tariff option is available independent of foreground SLAs.
dashboard/src/components/features/models/manage/ModelsContent.tsx	Surfaces background pricing independently and shows effective background pricing via 24h fallback when needed.
dashboard/src/api/control-layer/types.ts	Extends frontend types to recognize background as a completion window value.