feat: add import-institutions admin subcommand

What is this MR?

This MR introduces a new CLI capability to bulk import institutions from a CSV file into the system. It extends the existing admin command set by adding a dedicated subcommand for batch processing institutional data.

---

What does this MR do?

• Adds a new CLI command:

  corpus-client admin import-institutions <csv_path>

• Enables bulk creation of institutions by reading structured CSV input

• Validates file existence before processing

• Parses CSV rows using DictReader

• Normalizes and transforms data to match backend API expectations:

  • Converts numeric fields (e.g., college_code, revised_intake) to integers
  • Maps district names to standardized enum values
  • Normalizes categorical fields (e.g., college_type, management_type, medium, mode)
  • Detects CBCS in course names, extracts it into a flag, and cleans the name
  • Converts empty or placeholder values (-) to None

• Sends one API request per row to /api/v1/institutions

• Displays progress via spinner/progress indicator

• Outputs a summary of successful and failed imports

• Introduces _admin_request_silent helper for improved error handling during batch operations

Acceptance Criteria

• CLI command corpus-client admin import-institutions <csv_path> is available and functional
• Command exits with status code 1 and shows an error message if the file does not exist
• CSV rows are correctly parsed and transformed into API-compatible payloads
• Field normalization works correctly:
  • Numeric fields are properly converted
  • Enum and categorical values are standardized
  • CBCS detection and transformation is accurate
  • Empty or placeholder values are converted to None
• Each valid row triggers an API request to create an institution
• Progress indicator is shown during execution
• Final output includes success and failure counts
• Failures in individual rows do not interrupt the entire import process
• _admin_request_silent raises exceptions to allow controlled error handling

Test Coverage

• Added tests/test_import_institutions.py
• Covers:
  • Successful import of multiple CSV rows
  • Data normalization logic
  • CBCS handling behavior
  • Handling of missing or empty fields
  • Missing file scenario
• Note: Incomplete rows are processed with None values (not skipped), as validated by tests

---

Closes

• Closes #41 (closed)