Add editable trainer-since metadata - #62

Summary

Completes the reopened scope of #25 by making trainer tenure an explicit, editable profile value instead of deriving it permanently from account creation time.

• add a month/year Trainer since field to Account Settings
• persist the value independently from the instructor account audit timestamp
• return it from account creation, login, account refresh, and account update endpoints
• keep the browser session synchronized after edits and refreshes
• render the chosen value consistently on Trainer History
• centralize validation and local-safe month formatting in a shared helper

Why

Trainer History previously used instructors.created_at as a cosmetic proxy for tenure. That value answers when the application account was created, not when the instructor began training, so experienced trainers who joined the app later could not correct the displayed date.

The new trainer_since value uses month precision (YYYY-MM), matching the product requirement while preserving created_at unchanged for audit and debugging purposes.

Data migration

The schema adds an instructor_profiles table keyed by instructor ID.

• existing instructors are backfilled from the month portion of created_at
• new instructors receive an explicit trainer-since month during account creation
• INSERT OR IGNORE ... SELECT keeps the deployment schema idempotent
• account reads retain a created_at fallback for defensive compatibility
• updates use an upsert so profiles remain repairable if a legacy row is absent

A separate profile table avoids a non-idempotent ALTER TABLE in the schema file, which is intentionally rerun during every Worker deployment.

Client compatibility

Persisted browser sessions from before this field existed are normalized on load. If they contain the existing createdAt value, its month is used temporarily; the normal account refresh then replaces it with the server-authoritative profile value.

Date formatting constructs a local-noon calendar date from the year and month, avoiding UTC boundary shifts.

User impact

Instructors can now open Account Settings, choose the correct start month and year, and save it without changing their login name, profile photo, or account creation timestamp. Trainer History immediately reflects the corrected tenure.

Validation

• npm test — 24 tests passed
• npm run build — production TypeScript/Vite build passed
• npm run lint — passed with three pre-existing warnings
• npm --prefix worker run typecheck — passed
• git diff --check — passed
• schema creation, existing-instructor backfill, and repeated execution verified against an isolated in-memory SQLite database

The local Wrangler D1 runner could not execute on this Windows ARM64 host because its bundled workerd binary does not support that platform; the SQL itself was validated directly with SQLite instead.

Closes #25