Achievement badges + milestone credit bonuses

- Idempotent one-time credit bonus per milestone (payouts/firstBuyer/level2/level3): 10/25/50/100 cr
- Overview badges strip (unlockable medals) with canvas share-to-image (username overlaid)
- Newly-unlocked milestones toast the bonus on next dashboard load

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

db.js

@@ -70,6 +70,8 @@ async function bootstrap() {
     granted_welcome TINYINT NOT NULL DEFAULT 0,
     updated BIGINT NOT NULL
   ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4`);
+  const alterSafe0 = async sql => { try { await q(sql); } catch (e) { if (!['ER_DUP_FIELDNAME', 'ER_DUP_KEYNAME'].includes(e.code)) throw e; } };
+  await alterSafe0('ALTER TABLE earned_credits ADD COLUMN milestones VARCHAR(255) NULL'); // comma-joined granted milestone keys
   // additive columns (MySQL 8 has no IF NOT EXISTS for columns)
   const alterSafe = async sql => {
     try { await q(sql); }