feat: Add organization member support with memberType field

Implements support for both individual persons and organizations as members.

Changes:
- Added MemberType enum (INDIVIDUAL, ORGANIZATION) to distinguish member types
- Added member_type column to database (defaults to INDIVIDUAL for backward compatibility)
- Added company_name field for organizations
- Made first_name nullable (required for individuals, optional for organizations)
- Implemented conditional validation:
  - INDIVIDUAL members require first_name
  - ORGANIZATION members require company_name
- Updated GraphQL schema with new memberType and companyName fields
- Updated all resolvers to handle new fields and validation
- Added comprehensive unit tests for validation logic
- Updated existing tests to work with new fields
- All 53 tests passing

Technical notes:
- Using memberType instead of 'kind' to avoid GraphQL introspection conflicts
- Using native_enum=False for SQLite compatibility
- Using batch_alter_table for SQLite ALTER COLUMN compatibility
- Backward compatible: existing members automatically become INDIVIDUAL type

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

openspec/changes/support-organization-members/specs/organization-support/spec.md

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+# Spec: Organization Support
+
+## ADDED Requirements
+
+### Requirement: Distinguish member kind
+
+The system MUST allow defining a member as either an individual or an organization.
+
+#### Scenario: Create individual member
+**Given** client provides kind="INDIVIDUAL"
+**And** first_name="John"
+**When** createMember mutation executes
+**Then** member is created with kind INDIVIDUAL
+**And** first_name is "John"
+
+#### Scenario: Create organization member
+**Given** client provides kind="ORGANIZATION"
+**And** company_name="Acme Corp"
+**When** createMember mutation executes
+**Then** member is created with kind ORGANIZATION
+**And** company_name is "Acme Corp"
+
+### Requirement: Conditional mandatory fields
+
+The system MUST enforce different mandatory fields based on member kind.
+
+#### Scenario: Organization requires company name
+**Given** client provides kind="ORGANIZATION"
+**And** company_name is missing or empty
+**When** createMember mutation executes
+**Then** validation error is raised
+**And** error message indicates company_name is required for organizations
+
+#### Scenario: Organization allows missing first name
+**Given** client provides kind="ORGANIZATION"
+**And** company_name="Acme Corp"
+**And** first_name is missing
+**When** createMember mutation executes
+**Then** member is created successfully
+**And** first_name is null
+
+#### Scenario: Individual requires first name
+**Given** client provides kind="INDIVIDUAL"
+**And** first_name is missing
+**When** createMember mutation executes
+**Then** validation error is raised
+**And** error message indicates first_name is required for individuals
+
+## MODIFIED Requirements
+
+### Requirement: Member data structure
+
+The Member data structure MUST be modified to include kind and company_name, and allow nullable first_name.
+
+#### Scenario: Query member includes new fields
+**Given** member exists with kind="ORGANIZATION" and company_name="Acme Corp"
+**When** member query executes requesting kind and companyName
+**Then** kind is returned as "ORGANIZATION"
+**And** companyName is returned as "Acme Corp"