feat: remove birth_date field from Member resource

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

docs/database_schema.dbml

@@ -122,7 +122,6 @@ Table members {
   first_name text [not null, note: 'Member first name (min length: 1)']
   last_name text [not null, note: 'Member last name (min length: 1)']
   email text [not null, unique, note: 'Member email address (5-254 chars, validated)']
-  birth_date date [null, note: 'Date of birth (cannot be in future)']
   paid boolean [null, note: 'Payment status flag']
   phone_number text [null, note: 'Contact phone number (format: +?[0-9\- ]{6,20})']
   join_date date [null, note: 'Date when member joined club (cannot be in future)']
@@ -153,7 +152,7 @@ Table members {
     **Club Member Master Data**
     
     Core entity for membership management containing:
-    - Personal information (name, birth date, email)
+    - Personal information (name, email)
     - Contact details (phone, address)
     - Membership status (join/exit dates, payment status)
     - Additional notes
@@ -183,7 +182,6 @@ Table members {
     **Validation Rules:**
     - first_name, last_name: min 1 character
     - email: 5-254 characters, valid email format
-    - birth_date: cannot be in future
     - join_date: cannot be in future
     - exit_date: must be after join_date (if both present)
     - phone_number: matches pattern ^\+?[0-9\- ]{6,20}$