fix(db): widen customer_transactions.type to VARCHAR(128)

TransactionType.FIRST_SESSION_DISCOUNT ('first_session_discount', 22 chars) overflowed the VARCHAR(20) column, throwing in acceptPairingRequest AFTER the session was flipped to ACTIVE but before startSessionTimer/startSessionListener/PAIRED-notify ran. Every first-session-discount pairing thus half-completed: lost transaction row, no server-side timer, and a 500 to the mitra so its app never opened the chat. Widen the column (CREATE TABLE + idempotent ALTER). Deferred hardening (bookkeeping INSERT in the critical path) logged in TECH_DEBT.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/db/migrate.js

@@ -226,12 +226,19 @@ const migrate = async () => {
       id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
       customer_id UUID NOT NULL REFERENCES customers(id),
       session_id UUID NOT NULL REFERENCES chat_sessions(id),
-      type VARCHAR(20) NOT NULL,
+      type VARCHAR(128) NOT NULL,
       amount INT NOT NULL DEFAULT 0,
       created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
     )
   `
 
+  // Idempotent widen for DBs created when this column was VARCHAR(20): the
+  // TransactionType.FIRST_SESSION_DISCOUNT value 'first_session_discount' is
+  // 22 chars and overflowed varchar(20), throwing in acceptPairingRequest()
+  // *after* the session was already marked ACTIVE — losing the transaction row,
+  // the server-side timer, the PAIRED WS notify, and returning 500 to the mitra.
+  await sql`ALTER TABLE customer_transactions ALTER COLUMN type TYPE VARCHAR(128)`
+
   await sql`
     CREATE INDEX IF NOT EXISTS idx_customer_transactions_customer_id
     ON customer_transactions (customer_id)