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.

backend/src/models.py

@@ -24,6 +24,7 @@ class GenerateRequest(BaseModel):
     prompt: str
     model: str = DEFAULT_MODEL
     duration: Optional[int] = None  # defaults to model default
+    prompt_structure: Optional[dict] = None  # structured prompt fields for Pro mode
 
     @field_validator('model')
     @classmethod
@@ -40,6 +41,26 @@ class GenerateRequest(BaseModel):
         dur = self.duration if self.duration is not None else info["default_duration"]
         return min(dur, info["max_duration"])
 
+    def build_prompt(self) -> str:
+        """Build the final prompt from structured fields or return the simple prompt."""
+        if self.prompt_structure:
+            parts = ["Generate a track with the following elements:"]
+            field_order = [
+                ("style", "Style"),
+                ("timbre", "Timbre"),
+                ("structure", "Structure"),
+                ("mood", "Mood"),
+                ("tempo", "Tempo"),
+                ("instrumentation", "Instrumentation"),
+                ("effects", "Effects"),
+                ("additional", "Additional Notes"),
+            ]
+            for key, label in field_order:
+                if key in self.prompt_structure and self.prompt_structure[key]:
+                    parts.append(f"**{label}:** {self.prompt_structure[key]}")
+            return "\n\n".join(parts)
+        return self.prompt
+
 class GenerateResponse(BaseModel):
     id: str
     status: str
@@ -57,6 +78,9 @@ class Clip(BaseModel):
     status: str
     error_message: Optional[str] = None
     created_at: datetime
+    prompt_structure: Optional[dict] = None
+    bpm: Optional[float] = None
+    key: Optional[str] = None
 
 class RenameRequest(BaseModel):
     title: str