fix: SQL trigger-aware migration splitter, SpeechModel re-export, i64 type alignment

crates/storage/src/migrations.rs:
  Replace naive sql.split(';') with split_statements() that tracks
  BEGIN...END depth, so migrations containing trigger definitions
  execute correctly instead of being split mid-block.

crates/transcription/src/lib.rs:
  Re-export transcribe_rs::SpeechModel so callers in src-tauri can
  reference it without adding transcribe-rs as a direct dependency.

src-tauri/src/commands/models.rs:
  Use Box<dyn SpeechModel + Send> as the load_model_from_disk return
  type, matching the trait object that transcription crates produce.

src-tauri/src/commands/transcripts.rs:
  Remove the stale .map(|v| v as i32) casts on sample_rate and
  audio_channels. InsertTranscriptParams now stores these as i64
  (ebf449b), matching the i64 fields on CreateTranscriptRequest;
  casting to i32 first would silently truncate large sample rates.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/transcription/src/lib.rs

@@ -6,6 +6,7 @@ pub use concurrency::run_inference;
 pub use local_engine::{
     load_parakeet, load_whisper, LocalEngine, TimedTranscript,
 };
+pub use transcribe_rs::SpeechModel;
 pub use model_manager::{
     download, is_downloaded, list_downloaded, model_dir, models_dir,
 };