[codex] overhaul Freebuff premium sessions

[brandonkachen]

Summary

• Replace per-model Freebuff session caps with a shared premium-session pool: 5 premium sessions over 20 hours.
• Keep MiniMax unlimited while showing premium usage/unlimited status in the model picker, waiting room, and active status bar.
• Add fractional premium session accounting via free_session_admit.session_units, rounding early-ended active sessions up to the nearest 0.1 session.
• Update Freebuff session API tests and add the generated Drizzle migration.

Impact

Users can see how much premium usage they have consumed, can use MiniMax without session limits, and are not charged a full session when they end a premium session early. Users under the cap, including 4.9 / 5, can still start another premium session.

Validation

• bun test web/src/server/free-session/__tests__/public-api.test.ts
• bun test web/src/app/api/v1/freebuff/session/__tests__/session.test.ts
• bun test common/src/__tests__/freebuff-models.test.ts
• bun run --cwd common typecheck
• bun run --cwd web typecheck
• bun run --cwd cli typecheck
• git diff --check