feat(07.1-01): add user timezone column migration

- Adds `timezone TEXT NOT NULL DEFAULT 'UTC'` to "user" table (TZ-01)
- Backfills any NULL rows defensively
- Idempotent: ADD COLUMN IF NOT EXISTS, no destructive ops
- Storage timezone of existing TIMESTAMP columns unchanged

migrations/083_add_user_timezone.sql

@@ -0,0 +1,26 @@
+-- =============================================================================
+-- Per-user IANA timezone (Phase 7.1 — TZ-01)
+-- =============================================================================
+-- Adds a `timezone` column to the Better Auth "user" table so day/week
+-- boundary math (dashboards, ticket filters, finance, engagement) can be
+-- computed against the viewer's zone instead of server UTC.
+--
+-- Storage zone for every existing TIMESTAMP / TIMESTAMPTZ column is unchanged.
+-- Only display/range-bucketing logic in subsequent plans reads this column.
+--
+-- The SQL default here is the literal 'UTC'. The application-level default
+-- (process.env.DEFAULT_TIMEZONE || 'UTC') is enforced by Better Auth's
+-- additionalField `defaultValue` in lib/auth.ts so new sessions see the env-
+-- driven value even if a row was created without it.
+-- =============================================================================
+
+ALTER TABLE "user"
+  ADD COLUMN IF NOT EXISTS timezone TEXT NOT NULL DEFAULT 'UTC';
+
+-- Backfill any rows that may have been created with NULL (defensive — the
+-- DEFAULT clause above covers new inserts, but on managed Postgres a column
+-- added with DEFAULT may briefly show NULL in flight on some replicas).
+UPDATE "user" SET timezone = 'UTC' WHERE timezone IS NULL;
+
+COMMENT ON COLUMN "user".timezone IS
+  'IANA timezone string (e.g. America/New_York). Storage timezone for all date columns remains UTC; this column only affects display and range-bucketing.';