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/coderd.go

@@ -1139,6 +1139,13 @@ func New(options *Options) *API {
 			r.Post("/", api.postChats)
 			r.Get("/models", api.listChatModels)
 			r.Get("/watch", api.watchChats)
+			r.Route("/cost", func(r chi.Router) {
+				r.Get("/users", api.chatCostUsers)
+				r.Route("/{user}", func(r chi.Router) {
+					r.Use(httpmw.ExtractUserParam(options.Database))
+					r.Get("/summary", api.chatCostSummary)
+				})
+			})
 			r.Route("/files", func(r chi.Router) {
 				r.Use(httpmw.RateLimit(options.FilesRateLimit, time.Minute))
 				r.Post("/", api.postChatFile)