feat: add a privacy-safe local activity and processing history

[Abhash-Chakraborty]

Problem

The overhaul has operational state across uploads, processing jobs, archive/trash, vault, settings, and authentication, but no single local activity surface for understanding what changed or diagnosing failed work.

Acceptance criteria

• Define an append-only, user-scoped activity model with retention controls.
• Record privacy-safe events for upload/processing completion or failure, restore/archive/trash, vault lock/restore, and important settings changes.
• Never store image bytes, embeddings, OCR text, captions, secrets, or raw session tokens in activity payloads.
• Add a paginated/filterable API with shared-mode IDOR tests.
• Add a responsive timeline-style Activity route with empty, loading, error, and retry states.
• Add retention/clear controls and document local storage impact.
• Cover API, UI, accessibility, and migration rollback paths.

[payalrvs3]

Hey @Abhash-Chakraborty I'd like to take this up, please assign.