[Feature]: Notify users when file processing completes or fails

[bjrump]

Problem Statement

File processing runs as durable, asynchronous workflows (process_runs + worker), and a batch can take a long time. Today the only signal that processing finished — or that a file failed — is to open the project and look at the file status overview (#384, #428). A user who uploads a batch and walks away gets no cue when it's done, and a failed file (with its failure reason) goes unnoticed until they happen to look.

Proposed Solution

• Backend: on process_run completion/failure (and per-file failure), record a notification for the run's initiator. Add a lightweight count/list endpoint returning the current user's unseen processing events.
• Frontend: an in-app notification surface — a badge plus a short list of recent "processing complete" / "processing failed" events, each linking to the project or failed file. Refresh by polling and reuse the sidebar "unread" pattern in apps/frontend/components/sidebar/AppSidebar.tsx.

Acceptance Criteria

• When a user's upload batch finishes processing, they get an in-app notification linking to the project.
• When a file fails, the notification surfaces the failure reason and links to the failed file.
• The badge/count updates without a full reload (polling or invalidation).
• Notifications are scoped to the initiating user; no cross-user leakage.
• de + en strings.

Scope / Notes

• Sibling to [Feature]: Notification badge for pending Suggestions #464 (pending-Suggestions badge). Both are notification cues; keep them separate for now — a unified notification inbox is a larger, separate effort (as noted in [Feature]: Notification badge for pending Suggestions #464).
• In-app only for the MVP; email/push notifications are out of scope.
• Hooks into the existing workflow completion path (process_runs); no new worker concepts required.