feat: remove birth_date field from Member resource

Users who need birthday data can use custom fields instead.
Closes #161

docs/database-schema-readme.md

@@ -115,7 +115,6 @@ Member (1) → (N) Properties
 ### Member Constraints
 - First name and last name required (min 1 char)
 - Email unique, validated format (5-254 chars)
-- Birth date cannot be in future
 - Join date cannot be in future
 - Exit date must be after join date
 - Phone: `+?[0-9\- ]{6,20}`
@@ -169,7 +168,7 @@ Member (1) → (N) Properties
 ### Weighted Fields
 - **Weight A (highest):** first_name, last_name
 - **Weight B:** email, notes
-- **Weight C:** birth_date, phone_number, city, street, house_number, postal_code
+- **Weight C:** phone_number, city, street, house_number, postal_code
 - **Weight D (lowest):** join_date, exit_date
 
 ### Usage Example
@@ -381,7 +380,7 @@ Install "DBML Language" extension to view/edit DBML files with:
 - tokens (jti, purpose, extra_data)
 
 **Personal Data (GDPR):**
-- All member fields (name, email, birth_date, address)
+- All member fields (name, email, address)
 - User email
 - Token subject