[Week 04] Control API: start, stop, and status endpoints

[bobdingAI]

Tracker issue derived from the student's accepted proposal (proposal.pdf).

Week 04 goal (from proposal):

Weeks 3-5: Control API Implementation — "implement dataflow listing, loading, start, stop, and status endpoints; add basic error handling."

This issue covers the middle of the Weeks 3-5 block: implement the start/stop/status endpoints (POST /dataflows/{id}/start, POST /dataflows/{id}/stop, GET /dataflows/{id}/status) with basic error handling.

Definition of done: Backend can start and stop an example dataflow via HTTP and report its current lifecycle status.

Mentor: @bobdingAI
Student: @DGHX12345
Fork: https://github.com/DGHX12345/gsoc2026-dora-studio

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Refine scope with @bobdingAI in the first weekly meeting. Split into smaller issues if needed.

[DGHX12345]

Week 4 summarize:

• Extend RuntimeState with dataflowId/dataflowPath for selected flow tracking
• Add resolve_dataflow(id) and public DataflowFile resolver
• Add parser diagnostics for unsupported YAML sections
• Add start_dataflow(id, path, relative_path) with metadata tracking
• Add POST /api/dataflows/:id/start|stop|restart routes
• Update frontend with error fallback, dataflow selector, and selected-flow control
• 7 backend tests, frontend vue-tsc + vite build pass