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>

crates/core/src/lib.rs

@@ -9,5 +9,5 @@ pub mod types;
 pub use error::{KonError, Result};
 pub use types::{
     AudioSamples, DownloadProgress, EngineName, Megabytes, ModelId, Segment,
-    Transcript, TranscriptionOptions,
+    Transcript, TranscriptMetadata, TranscriptionOptions,
 };

crates/core/src/types.rs

@@ -166,6 +166,21 @@ pub struct TranscriptionOptions {
     pub initial_prompt: Option<String>,
 }
 
+/// Full provenance metadata for a transcript.
+/// Captures everything needed to reproduce the transcription.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct TranscriptMetadata {
+    pub engine: String,
+    pub model_id: ModelId,
+    pub inference_ms: u64,
+    pub sample_rate: u32,
+    pub audio_channels: u16,
+    pub format_mode: String,
+    pub remove_fillers: bool,
+    pub british_english: bool,
+    pub anti_hallucination: bool,
+}
+
 /// Progress update during model download.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct DownloadProgress {