feat: unify importer and scrobbler APIs under a single app

[tsshadow]

Summary

• move the importer endpoints and websocket handlers into a dedicated APIRouter module and reuse it across services
• add a new unified FastAPI application that hosts the scrobbler routers alongside the importer API and lifecycle hooks
• refresh Docker setup, entrypoints, and documentation to reference the combined backend service

Testing

• not run (not requested)

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Move importer API under new prefix without updating frontend defaults

The new router now mounts every importer endpoint and WebSocket under /importer (APIRouter(prefix="/importer")). The Svelte dashboard that ships in modules/importer/frontend still defaults VITE_API_BASE to /api and constructs the WebSocket URL as /ws/jobs, so a stock build will keep calling the old paths and receive 404s. Unless every deployment sets a custom VITE_API_BASE at build time, the importer UI can no longer reach the backend. Consider either exposing the routes under both prefixes for compatibility or updating the frontend defaults to /importer/api and /importer/ws/jobs in the same change.

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