feat: Structured prompt editing for Pro model

Backend:
- Added prompt_structure JSON column to clips DB (migration in init_db)
- Updated GenerateRequest model with optional prompt_structure dict
- build_prompt() method combines structured fields into single prompt:
  'Generate a track with the following elements:\n\n**Style:** [style]\n...'
- create_clip() accepts and stores prompt_structure JSON
- get_clips()/get_clip() parse prompt_structure back to dict

Frontend:
- New StructuredPromptEditor component with 8 collapsible fields:
  Style, Timbre, Structure, Mood, Tempo, Instrumentation, Effects, Additional
  - Fields show filled/empty state with colored dots
  - Expand/collapse individual fields
  - 'Expand All' button
  - Counter: 'X of 8 fields filled'
- Generator component updated with Basic/Advanced mode toggle (Pro only)
  - Basic: single textarea (works for both models)
  - Advanced: StructuredPromptEditor (Pro only)
  - Mode resets to Basic when switching to Clip Preview
- DetailPanel shows structured prompt breakdown when present
  - Each filled field shown in its own labeled box
  - Style, Timbre, Structure, Mood, Tempo, Instrumentation, Effects, Additional
- Clip interface updated with prompt_structure field across all components

Architecture KB and Tasks KB updated.

frontend/src/App.tsx

@@ -18,6 +18,7 @@ interface Clip {
   artwork_filename?: string | null
   bpm?: number | null
   key?: string | null
+  prompt_structure?: Record<string, string> | null
   created_at: string
 }