feat: add chat cost analytics backend (#23036)

Add cost tracking for LLM chat interactions with microdollar precision.

## Changes
- Add `chatcost` package for per-message cost calculation using
`shopspring/decimal` for intermediate arithmetic
- **Ceil rounding policy**: fractional micros round UP to next whole
micro (applied once after summing all components)
- Database migration: `total_cost_micros` BIGINT column with historical
backfill and `created_at` index
- API endpoints: per-user cost summary and admin rollup under
`/api/experimental/chats/cost/`
- SDK types: `ChatCostSummary`, `ChatCostModelBreakdown`,
`ChatCostUserRollup`
- Fix `modeloptionsgen` to handle `decimal.Decimal` as opaque numeric
type
- Update frontend pricing test fixtures for string decimal types

## Design decisions
- `NULL` = unpriced (no matching model config), `0` = free
- Reasoning tokens included in output tokens (no double-counting)
- Integer microdollars (BIGINT) for storage and API responses
- Price config uses `decimal.Decimal` for exact parsing; totals use
`int64`

Frontend: #23037

coderd/database/models.go

@@ -4020,6 +4020,7 @@ type ChatMessage struct {
 	Compressed          bool                  `db:"compressed" json:"compressed"`
 	CreatedBy           uuid.NullUUID         `db:"created_by" json:"created_by"`
 	ContentVersion      int16                 `db:"content_version" json:"content_version"`
+	TotalCostMicros     sql.NullInt64         `db:"total_cost_micros" json:"total_cost_micros"`
 }
 
 type ChatModelConfig struct {