chore: Convert field_type from VARCHAR to PostgreSQL ENUM

[xernobyl]

Summary

Converts the field_type column from VARCHAR to a PostgreSQL ENUM type, and implements a type-safe string-based enum in Go to ensure only valid field types can be used.

Changes

Database Schema

• sql/001_initial_schema.sql: Updated to use field_type_enum instead of VARCHAR

Go Models (internal/models/feedback_records.go)

• Added FieldType string-based enum type with constants
• Implemented ValidFieldTypes map using map[FieldType]struct{} for efficient set membership checks
• Added IsValid() and ParseFieldType() methods
• Added UnmarshalJSON() for JSON validation
• Updated structs to use FieldType instead of string:
  • FeedbackRecord.FieldType
  • CreateFeedbackRecordRequest.FieldType
  • ListFeedbackRecordsFilters.FieldType

Validation (internal/api/validation/validation.go)

• Updated validateFieldType to use models.ParseFieldType() and ValidFieldTypes map
• Added form decoder support for *FieldType in query parameters

OpenAPI Specification

• Added enum values to field_type in response schema
• Added enum values to field_type query parameter
• Request schema already had correct enum values

Benefits

• Type Safety: Compile-time and runtime validation ensures only valid field types
• Data Integrity: PostgreSQL ENUM enforces constraints at database level
• Better Documentation: OpenAPI spec clearly shows valid values
• Performance: Efficient set membership checks using struct{} map values