Add page bootstrap BFF endpoint to collapse per-page API fan-out

[amondnet]

Sub-task of #3 (Fix B).

Each page fires ~8 parallel API calls, each independently building a scoped executor (packages/core/api/src/server/scoped-executor.ts:253).

Tasks:

• Add a single aggregated endpoint that authenticates once, builds one scoped executor, and returns account + integrations + tools + policies + connections.
• Switch the policies / integrations pages to a single call.
• Expected warm data-complete: ~1.77s to ~0.6-0.9s.

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