Closes #28637

Adds a usage_by_model TEXT JSON column to the sessions table that
records per-model token/cost breakdown on every API call, enabling
accurate cost attribution and display after mid-session /model
switches.

Changes

• Schema: usage_by_model TEXT column on sessions, auto-added to
  existing DBs via _reconcile_columns(), bump SCHEMA_VERSION to 12
• In-memory: agent.session_usage_by_model dict on AIAgent
• Per-call tracking: populates per-model counters in
  conversation_loop.py — tokens, cache, reasoning, API calls, and
  cost — then serializes to JSON and persists on every turn
• CLI /usage: per-model breakdown when multiple models detected;
  single-model fallback for legacy sessions
• Gateway /usage: same breakdown in messaging platforms
• /insights: model breakdown uses usage_by_model when present,
  falls back to model column for backward compat
• TUI: usage_by_model included in Python → TypeScript payload
  (display rendering not yet implemented — separate follow-up)
• Tests: round-trip serialization, cumulative sum invariant, backward
  compat fallback, schema version bump

Backward compatibility

• Old sessions without the column render single-model as before
• _reconcile_columns() adds the column on next startup — zero-downtime
• Cumulative counters (input_tokens, output_tokens, etc.) are
  unchanged