feat: add indication pathway processing functions (Task 2.3)

- Add generate_icicle_chart_indication() to pathway_analyzer.py
  - Variant that uses indication_df instead of directory_df
  - Groups by Trust → Search_Term → Drug → Pathway
  - Accepts indication_df mapping UPID → Indication_Group

- Add process_indication_pathway_for_date_filter() to pathway_pipeline.py
  - Processes indication-based pathway for a single date filter
  - Uses generate_icicle_chart_indication() for hierarchy building

- Add extract_indication_fields() to pathway_pipeline.py
  - Extracts trust_name, search_term, drug_sequence from ids column
  - Similar to extract_denormalized_fields() but for indication charts

- Update convert_to_records() with chart_type parameter
  - Includes chart_type column in output records
  - Supports "directory" and "indication" values

- Add ChartType type alias (Literal["directory", "indication"])

- Update __all__ exports with new functions

IMPLEMENTATION_PLAN.md

@@ -90,13 +90,18 @@ python -m reflex compile
 - [x] Verify: Migration adds column, existing data defaults to "directory"
 
 ### 2.3 Create Indication Pathway Processing
-- [ ] Add `process_indication_pathways()` to `pathway_pipeline.py`:
+- [x] Add `process_indication_pathway_for_date_filter()` to `pathway_pipeline.py`:
   - Group by: Trust → Search_Term → Drug → Pathway
   - For unmatched patients: use directorate name as Search_Term fallback
   - Output: Same structure as directory pathways but with indication grouping
-- [ ] Add `extract_indication_fields()` for denormalized columns:
+- [x] Add `generate_icicle_chart_indication()` to `pathway_analyzer.py`:
+  - Variant of `generate_icicle_chart()` that uses indication_df instead of directory_df
+  - Takes `indication_df` parameter mapping UPID → Indication_Group
+- [x] Add `extract_indication_fields()` for denormalized columns:
   - Extract: trust_name, search_term (or fallback_directorate), drug_sequence
-- [ ] Verify: Process sample data, check hierarchy structure
+- [x] Update `convert_to_records()` to include `chart_type` parameter
+- [x] Add `ChartType` type alias ("directory" | "indication")
+- [x] Verify: Code compiles, imports work correctly
 
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