fix(insights): derive briefing warm budget from the response timeout

The comprehensive briefing warm/cron path wrapped generation in a fixed
130 s outer abort, but the per-user response-timeout setting accepts
10-600 s and the AI settings tell users to raise it for slow local
models. Any account whose response timeout was >= ~130 s had every
comprehensive warm clipped by the warm bound, not the provider, so the
generation never landed: no cached briefing row was written and the
dashboard fell to the hard empty state.

Size the warm outer abort from the resolved effective timeout
(`aiResponseTimeoutSeconds * 1000`, falling back to the comprehensive
surface budget) plus a fixed headroom for feature extraction, prompt
assembly, and the cache write. A raised setting now extends the warm and
nightly budgets instead of being silently capped. An absolute ceiling
(the 600 s write-time maximum plus the headroom) keeps a wedged call from
pinning the worker while staying above the largest value a user can set.
Both the nightly loop and the forced on-visit warm read the per-user
setting and use the derived budget.

Make the empty state more useful: record the upstream HTTP status on a
failure marker and classify it (timeout / auth / rate-limit / provider /
format), so the "couldn't generate" state points at the right lever —
raise the response timeout for a timeout, re-check the provider for an
auth rejection — instead of one generic line. Keep-last-good behaviour is
unchanged.

Align the response-timeout field copy with the actual default budget
(~120 s, not ~180 s) across the placeholder, the OpenAPI description, and
the inline note.

docs/api/openapi.yaml

@@ -16947,8 +16947,8 @@ components:
               minimum: -9007199254740991
               maximum: 9007199254740991
             - type: "null"
-          description: v1.22 (#89) — per-user response timeout for AI generation, in seconds (10–600). Null = the generous
-            built-in default (~180 s). Mainly for slow local/self-hosted backends.
+          description: v1.22 (#89) — per-user response timeout for AI generation, in seconds (10–600). Null = the built-in
+            comprehensive-briefing default (~120 s). Mainly for slow local/self-hosted backends.
         ocrEnabled:
           type: boolean
           description: v1.22 (#90) — whether a dedicated document-scan (Lab-OCR) provider is active. When false the OCR path uses