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/design.md

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+# Design: Organization Member Support
+
+## Data Model Changes
+
+### Member Table
+- Add `member_type` column: Enum (`INDIVIDUAL`, `ORGANIZATION`). Default to `INDIVIDUAL` for existing records.
+- Add `company_name` column: String, nullable.
+- Alter `first_name`: Change from `nullable=False` to `nullable=True`.
+
+### Constraints
+- **Application Level**:
+    - If `member_type` is `INDIVIDUAL`: `first_name` must be present.
+    - If `member_type` is `ORGANIZATION`: `company_name` must be present.
+- **Database Level**:
+    - We can't easily enforce conditional not-null constraints in standard SQL without triggers or check constraints. For simplicity and portability (SQLite/Postgres), we will enforce this at the application level (Pydantic/SQLAlchemy validators) and potentially add a CHECK constraint if supported by both dialects easily.
+    - `first_name` will become nullable in the DB schema.
+
+## API Changes
+
+### GraphQL Schema
+- Update `Member` type to include `memberType` and `companyName`.
+- Update `createMember` and `updateMember` inputs.
+- Add validation logic in resolvers to enforce the conditional requirements.
+
+## Migration Strategy
+1. Add `member_type` column with default 'INDIVIDUAL'.
+2. Add `company_name` column.
+3. Alter `first_name` to be nullable.
+
+## Technical Note
+- Using `memberType` instead of `kind` to avoid conflicts with GraphQL introspection queries which use `kind` as a metadata field.