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>

TECH_DEBT.md

@@ -10,6 +10,30 @@ to act on it without re-deriving the discussion.
 
 ## Backend
 
+### `[2026-06-01]` Bookkeeping INSERT sits in the pairing critical path
+
+**File:** `backend/src/services/pairing.service.js` (`acceptPairingRequest`, ~line 506)
+
+**What happened:** the `INSERT INTO customer_transactions` runs *after* the session
+is flipped to `ACTIVE` but *before* `startSessionTimer`, `startSessionListener`,
+the customer `PAIRED` WS notify, and the other-mitra dismiss fan-out. A
+`varchar(20)` overflow on `type = 'first_session_discount'` (22 chars) threw
+there, so every first-session-discount pairing half-completed: no transaction
+row, no server-side timer, no PAIRED push (customer recovered via polling), and a
+500 returned to the mitra so its app never opened the chat.
+
+**Fixed now:** column widened to `VARCHAR(128)` (migrate.js), so the INSERT no
+longer throws.
+
+**Why it's still debt:** a *bookkeeping* write can still abort *critical* pairing
+steps if it ever fails again (constraint change, DB hiccup, future longer enum).
+Hardening: either move the `customer_transactions` INSERT to the end of
+`acceptPairingRequest`, or wrap it in a `try/catch` that logs-but-doesn't-throw,
+so transaction recording can never again half-complete a pairing. Same applies to
+the equivalent INSERT in `extension.service.js`.
+
+---
+
 ### `[2026-05-11]` Public `GET /api/public/bestie/available` needs rate limiting before prod
 
 **File:** `backend/src/routes/public/public.bestie-availability.routes.js`

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)