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/storage/src/lib.rs

@@ -4,6 +4,6 @@ pub mod file_storage;
 pub use database::{
     complete_task, delete_task, delete_transcript, get_setting, get_transcript,
     init, insert_task, insert_transcript, list_tasks, list_transcripts,
-    set_setting, TaskRow, TranscriptRow,
+    log_error, set_setting, InsertTranscriptParams, TaskRow, TranscriptRow,
 };
 pub use file_storage::{app_data_dir, database_path, recordings_dir};