refactor: slim pathways.db from 351 MB to 3.5 MB by removing unused tables

Drop fact_interventions (440K rows), mv_patient_treatment_summary (35K rows),
ref_drug_snomed_mapping (144K rows), and processed_files — all unused since
the app moved to pre-computed pathway_nodes.

Key changes:
- Rewrite load_data() to source from pathway_nodes + pathway_refresh_log
- Remove 7 dead methods and 8 dead state vars from pathways_app.py
- Delete patient_data.py, load_snomed_mapping.py, test_large_dataset_performance.py
- Remove SQLiteDataLoader (depended on fact_interventions)
- Remove file tracking schema (processed_files tracked fact_interventions loads)
- Remove legacy diagnosis functions from diagnosis_lookup.py
- Add source_row_count migration for pathway_refresh_log
- Clean all cross-references in __init__.py, data_source.py, migrate.py

cli/refresh_pathways.py

@@ -176,6 +176,7 @@ def log_refresh_complete(
     record_count: int,
     date_filter_counts: dict[str, int],
     duration_seconds: float,
+    source_row_count: Optional[int] = None,
 ) -> None:
     """Log the successful completion of a refresh operation."""
     conn.execute("""
@@ -184,13 +185,15 @@ def log_refresh_complete(
             status = 'completed',
             record_count = ?,
             date_filter_counts = ?,
-            processing_duration_seconds = ?
+            processing_duration_seconds = ?,
+            source_row_count = ?
         WHERE refresh_id = ?
     """, (
         datetime.now().isoformat(),
         record_count,
         json.dumps(date_filter_counts),
         duration_seconds,
+        source_row_count,
         refresh_id,
     ))
     conn.commit()
@@ -517,6 +520,7 @@ def refresh_pathways(
                 record_count=stats["total_records"],
                 date_filter_counts=stats["date_filter_counts"],
                 duration_seconds=elapsed,
+                source_row_count=stats.get("snowflake_rows"),
             )
 
             # Verify final counts