feat(pricing): move model pricing to a DB-backed, periodically-updated table #22

Context

`cli/src/utils/pricing.ts` currently hardcodes a static `MODEL_PRICING` map (Claude, Gemini, OpenAI/Codex) compiled into the CLI. Any time a provider changes pricing or ships a new model, the fix requires a code change + release (see the recent Codex GPT-5.6 pricing fix — cost was hardcoded to 0 until manually patched).

Proposal

Move model pricing out of source code and into the local SQLite database (~/.code-insights/data.db), with a mechanism to refresh it periodically instead of requiring a CLI update:

• New table (e.g. model_pricing) keyed by (model_id, effective_from) so historical sessions are costed with the price that was actually in effect at the time, not today's price.
• A sync/update path (e.g. code-insights sync or a dedicated code-insights pricing update command) that refreshes the table from a maintained source, with the current hardcoded map kept as the offline/bundled fallback seed.
• getModelPricing() / calculateCost() in pricing.ts read from the DB table first, falling back to the bundled defaults if the DB has no entry or the model is unrecognized.

Why

• New models currently silently cost $0 or fall back to a generic default until someone notices and patches pricing.ts in a release.
• Historical cost figures are wrong once a model's price changes, since the static table only ever holds "current" pricing — no way to reprice old sessions accurately or preserve what a session cost at the time.

Scope note

Needs a design decision on where the periodic price data comes from (bundled JSON updated per release vs. remote fetch vs. manual entry) before implementation — flagging for scoping, not diving into full build yet.

Related development (managed by Project #6 audit)

• PR code-insights#23 (merged)