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>

migrations/versions/b61b6b2d83cc_add_organization_member_support.py

@@ -0,0 +1,57 @@
+"""add organization member support
+
+Revision ID: b61b6b2d83cc
+Revises: 0f79356844b5
+Create Date: 2025-12-04 12:03:16.548989
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'b61b6b2d83cc'
+down_revision: Union[str, Sequence[str], None] = '0f79356844b5'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # Add member_type column with default 'INDIVIDUAL'
+    op.add_column(
+        'members',
+        sa.Column('member_type', sa.String(length=20), nullable=False, server_default='INDIVIDUAL')
+    )
+
+    # Add company_name column (nullable)
+    op.add_column(
+        'members',
+        sa.Column('company_name', sa.String(length=200), nullable=True)
+    )
+
+    # Alter first_name to be nullable (SQLite compatible using batch operations)
+    with op.batch_alter_table('members', schema=None) as batch_op:
+        batch_op.alter_column(
+            'first_name',
+            existing_type=sa.String(length=100),
+            nullable=True
+        )
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # Remove company_name and member_type columns
+    op.drop_column('members', 'company_name')
+    op.drop_column('members', 'member_type')
+
+    # Revert first_name to nullable=False
+    # Note: This may fail if there are organization members without first_name
+    with op.batch_alter_table('members', schema=None) as batch_op:
+        batch_op.alter_column(
+            'first_name',
+            existing_type=sa.String(length=100),
+            nullable=False
+        )