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/components/StructuredPromptEditor.tsx

@@ -0,0 +1,106 @@
+import { useState } from 'react'
+
+export interface PromptStructure {
+  style?: string
+  timbre?: string
+  structure?: string
+  mood?: string
+  tempo?: string
+  instrumentation?: string
+  effects?: string
+  additional?: string
+}
+
+interface Props {
+  value: PromptStructure
+  onChange: (value: PromptStructure) => void
+}
+
+const FIELDS = [
+  { key: 'style', label: 'Style / Genre', placeholder: 'e.g., Gabber, Ambient, Orchestral, Phonk...' },
+  { key: 'timbre', label: 'Timbre / Texture', placeholder: 'e.g., Warm analog synths, gritty distortion, ethereal pads...' },
+  { key: 'structure', label: 'Structure', placeholder: 'e.g., Intro-Verse-Chorus-Verse-Chorus-Bridge-Chorus-Outro' },
+  { key: 'mood', label: 'Mood / Energy', placeholder: 'e.g., Dark and aggressive, euphoric, melancholic, triumphant...' },
+  { key: 'tempo', label: 'Tempo / BPM', placeholder: 'e.g., 180 BPM, slow build to 200 BPM, variable...' },
+  { key: 'instrumentation', label: 'Instrumentation', placeholder: 'e.g., Heavy kick drums, distorted leads, vocal chops, sub-bass...' },
+  { key: 'effects', label: 'Effects / Processing', placeholder: 'e.g., Heavy sidechain compression, reverb tails, bitcrush...' },
+  { key: 'additional', label: 'Additional Notes', placeholder: 'Any other details, references, or specific instructions...' },
+] as const
+
+export default function StructuredPromptEditor({ value, onChange }: Props) {
+  const [expanded, setExpanded] = useState<Set<string>>(new Set(['style', 'structure', 'mood']))
+
+  const toggleField = (key: string) => {
+    setExpanded(prev => {
+      const next = new Set(prev)
+      if (next.has(key)) {
+        next.delete(key)
+      } else {
+        next.add(key)
+      }
+      return next
+    })
+  }
+
+  const updateField = (key: string, text: string) => {
+    onChange({ ...value, [key]: text })
+  }
+
+  const filledCount = FIELDS.filter(f => value[f.key as keyof PromptStructure]?.trim()).length
+
+  return (
+    <div className="space-y-2">
+      <div className="flex items-center justify-between">
+        <span className="text-xs text-lyria-muted">
+          {filledCount} of {FIELDS.length} fields filled
+        </span>
+        <button
+          onClick={() => setExpanded(new Set(FIELDS.map(f => f.key)))}
+          className="text-xs text-lyria-accent hover:text-lyria-accent/80 transition-colors"
+        >
+          Expand All
+        </button>
+      </div>
+
+      {FIELDS.map(({ key, label, placeholder }) => {
+        const isExpanded = expanded.has(key)
+        const hasValue = value[key as keyof PromptStructure]?.trim()
+
+        return (
+          <div key={key} className="border border-white/5 rounded-lg overflow-hidden">
+            <button
+              onClick={() => toggleField(key)}
+              className={`w-full flex items-center justify-between px-3 py-2 text-sm transition-colors ${
+                hasValue ? 'bg-white/5' : 'bg-transparent hover:bg-white/5'
+              }`}
+            >
+              <div className="flex items-center gap-2">
+                <span className={`w-1.5 h-1.5 rounded-full ${hasValue ? 'bg-lyria-accent' : 'bg-lyria-muted/30'}`} />
+                <span className={hasValue ? 'text-white' : 'text-lyria-muted'}>{label}</span>
+              </div>
+              <svg
+                className={`w-3.5 h-3.5 text-lyria-muted transition-transform ${isExpanded ? 'rotate-180' : ''}`}
+                fill="none"
+                stroke="currentColor"
+                viewBox="0 0 24 24"
+              >
+                <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M19 9l-7 7-7-7" />
+              </svg>
+            </button>
+
+            {isExpanded && (
+              <div className="px-3 pb-3">
+                <textarea
+                  value={value[key as keyof PromptStructure] || ''}
+                  onChange={(e) => updateField(key, e.target.value)}
+                  placeholder={placeholder}
+                  className="w-full h-20 bg-black/20 border border-white/5 rounded-md px-3 py-2 text-sm text-white placeholder:text-lyria-muted/30 focus:outline-none focus:border-lyria-accent/50 resize-none"
+                />
+              </div>
+            )}
+          </div>
+        )
+      })}
+    </div>
+  )
+}