fix(ai): keep background work off a benched provider and charge failed replies

The health ledger already counted every failed call and paused a provider
that kept failing, with a pause that grows. A paused provider only moved to
the tail of the chain, though, and the tail is still walked: a chain whose
only entry answered every call with reasoning and no answer was called by
every background job regardless.

A walk for background work (surface "job") now leaves out every provider
the ledger has benched until its window lifts; the local floor only once
it is paused, past a brief backoff. One ai.chain.background_paused line
per window says so. A person's own call still reaches the provider at the
tail and reports its own error. The ledger is in Postgres, so the bench
holds across workers and restarts.

The status, briefing and document-index paths refunded a failed call in
full, so background spend on replies without an answer never reached the
day's token ceiling. A failed chain now carries the tokens its hops
reported (tokensSpent) and those paths charge them.

The two clients share one answerless step (askAgainUnreasoned), which
keeps both inside the source size ratchet; its baselines go down.

Refs #1279

src/__tests__/baselines/source-function-lines.json

@@ -1,7 +1,7 @@
 {
   "$comment": "Production functions over 200 lines, keyed \"file :: name\", and their length. Maintained by src/__tests__/source-size-ratchet.test.ts.",
   "count": 352,
-  "total": 113949,
+  "total": 113900,
   "entries": {
     "app/achievements/page.tsx :: AchievementsPage": 206,
     "app/api/admin/backups/upload/route.ts :: apiHandler(arg)": 314,
@@ -255,8 +255,8 @@
     "lib/ai/coach/turn/pipeline.ts :: runTurn": 274,
     "lib/ai/coach/turn/reply-guards.ts :: guardReply": 331,
     "lib/ai/codex-client.ts :: parseResponse": 302,
-    "lib/ai/local-client.ts :: generateCompletionStream": 298,
-    "lib/ai/openai-client.ts :: generateCompletion": 375,
+    "lib/ai/local-client.ts :: generateCompletionStream": 294,
+    "lib/ai/openai-client.ts :: generateCompletion": 335,
     "lib/analytics/bp-in-target-fast-path.ts :: computeFromRollups": 203,
     "lib/analytics/compliance/display.ts :: calculateCompliance": 212,
     "lib/analytics/correlations-fast-path.ts :: computeCorrelationHypothesesFastPath": 256,
@@ -299,7 +299,7 @@
     "lib/insights/comprehensive-generate.ts :: generateComprehensiveInsight": 876,
     "lib/insights/features.ts :: extractFeatures": 796,
     "lib/insights/mood-aggregates.ts :: fetchMoodAggregates": 246,
-    "lib/insights/status-provider.ts :: runStatusCompletion": 241,
+    "lib/insights/status-provider.ts :: runStatusCompletion": 236,
     "lib/integrations/connection-health-load.ts :: loadConnectionFacts": 227,
     "lib/integrations/status.ts :: recordSyncFailure": 292,
     "lib/jobs/apple-health-import-worker.ts :: handleAppleHealthImport": 300,