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/sqlc.yaml

@@ -148,6 +148,17 @@ sql:
             go_type: "database/sql.NullTime"
           - column: "task_event_data.first_status_after_resume_at"
             go_type: "database/sql.NullTime"
+          - db_type: "pg_catalog.numeric"
+            go_type:
+              import: "github.com/shopspring/decimal"
+              type: "Decimal"
+              package: "decimal"
+          - db_type: "pg_catalog.numeric"
+            nullable: true
+            go_type:
+              import: "github.com/shopspring/decimal"
+              type: "NullDecimal"
+              package: "decimal"
         rename:
           group_member: GroupMemberTable
           group_members_expanded: GroupMember