feat(kon): add full transcript provenance — enriched schema, metadata pipeline, error log

Schema enrichment:
- transcripts table: +engine, +model_id, +inference_ms, +sample_rate, +audio_channels,
  +format_mode, +remove_fillers, +british_english, +anti_hallucination
- New error_log table: context, error_code, message, metadata (JSON)
- 5 indices on frequently queried columns

Metadata pipeline:
- TranscriptMetadata struct in kon-core for full provenance capture
- LocalEngine tracks loaded ModelId, returns TimedTranscript with inference_ms
- std::time::Instant timing around transcribe-rs inference calls
- Tauri transcription commands emit inference_ms in event payloads
- InsertTranscriptParams accepts all provenance fields
- log_error() for structured error logging to SQLite

Every transcript is now fully reproducible from its stored metadata.
23 tests passing, clippy clean.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/src/commands/models.rs

@@ -84,10 +84,11 @@ pub async fn load_model(
     }
 
     let engine = state.whisper_engine.clone();
+    let mid = id.clone();
     tokio::task::spawn_blocking(move || {
         let model = kon_transcription::load_whisper(&model_file)
             .map_err(|e| e.to_string())?;
-        engine.load(model);
+        engine.load(model, mid);
         Ok::<_, String>(())
     })
     .await
@@ -152,10 +153,11 @@ pub async fn load_parakeet_model(
     }
 
     let engine = state.parakeet_engine.clone();
+    let mid = id.clone();
     tokio::task::spawn_blocking(move || {
         let model = kon_transcription::load_parakeet(&dir)
             .map_err(|e| e.to_string())?;
-        engine.load(model);
+        engine.load(model, mid);
         Ok::<_, String>(())
     })
     .await