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/transcription/src/lib.rs

@@ -3,5 +3,9 @@ pub mod local_engine;
 pub mod model_manager;
 
 pub use concurrency::run_inference;
-pub use local_engine::{load_parakeet, load_whisper, LocalEngine};
-pub use model_manager::{download, is_downloaded, list_downloaded, model_dir, models_dir};
+pub use local_engine::{
+    load_parakeet, load_whisper, LocalEngine, TimedTranscript,
+};
+pub use model_manager::{
+    download, is_downloaded, list_downloaded, model_dir, models_dir,
+};