feat(store): add users.email_verified_at column + model plumbing (TASK-1935)

[xarmian]

Wave 1 of PLAN-1933 (Pad Cloud email verification). Pure infra, no behavior change — nothing reads the column until Wave 3, so this is mergeable early behind the (not-yet-mounted) middleware.

What

• Migration 070_email_verified.sql (SQLite) + 048_email_verified.sql (Postgres): add nullable users.email_verified_at TEXT, mirroring disabled_at. SQLite ALTER TABLE ADD COLUMN without IF NOT EXISTS (SQLite rejects it here); Postgres with IF NOT EXISTS.
• UNCONDITIONAL backfill: every existing row is set to verified (RFC3339 Z-suffixed via strftime / to_char) so no existing / OAuth / self-host account is write-locked on deploy. This is inverted vs password_set's conditional (OAuth-aware) backfill — there is no "was this user ever verified?" signal, and the correct answer for every pre-existing account is "verified".
• SAFE default = verified (DR-3): CreateUser / CreateOAuthUser write a verified timestamp unless a new explicit UserCreate.Unverified field requests otherwise. Only the future cloud self-serve branch (Wave 3) will set it, so a missed call site fails SAFE (verified), not write-locked.
• Model: models.User.EmailVerifiedAt + IsEmailVerified() (mirrors IsDisabled()); UserCreate.Unverified (zero value → verified).
• Both scan sites updated: scanUser AND the inline scan in SearchUsers (missing the second breaks the admin user list at runtime with a column/target mismatch).
• Session payload: derived email_verified bool in sessionUserPayload for a later web wave.

Tests

• Backfill verifies pre-existing NULL rows (runs the real migration DML, both dialects).
• Fresh CreateUser verified by default; explicit Unverified: true persists as NULL; OAuth users verified.
• Both scan paths (scanUser + SearchUsers) return the field for verified and unverified users.
• IsEmailVerified() unit test.

Gates

• make check — green.
• make test-pg — green (dual-dialect verified; SQLite passing does not prove Postgres).
• Codex review — CLEAN.

https://claude.ai/code/session_01HxBkAMiFBtCRJ2tKSCt3ST