feat: replace age integer with date_of_birth across backend and client

Store date_of_birth (DATE) instead of a static age integer so that age
is always computed dynamically from the stored date of birth.

- Migration 011: adds date_of_birth, backfills from age, drops age
- AgeFromDOB helper computes current age from YYYY-MM-DD string
- User model, repository SQL, and service validation updated
- Flutter: User.age becomes a computed getter; profile edit screen
  uses a date picker bounded to [today-120y, today-10y]

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/011_replace_age_with_date_of_birth.sql

@@ -0,0 +1,13 @@
+-- +goose Up
+ALTER TABLE users ADD COLUMN date_of_birth DATE;
+UPDATE users
+    SET date_of_birth = (CURRENT_DATE - (age * INTERVAL '1 year'))::DATE
+    WHERE age IS NOT NULL;
+ALTER TABLE users DROP COLUMN age;
+
+-- +goose Down
+ALTER TABLE users ADD COLUMN age SMALLINT;
+UPDATE users
+    SET age = EXTRACT(YEAR FROM AGE(CURRENT_DATE, date_of_birth))::SMALLINT
+    WHERE date_of_birth IS NOT NULL;
+ALTER TABLE users DROP COLUMN date_of_birth;