feat(database,webapp): add LlmModel pricing_unit column and admin selector (#3820)

## Summary

Adds a nullable `pricing_unit` column to the LLM model registry's
`llm_models` table, recording how each model is billed ("tokens",
"characters", "images", "minutes", "requests", "free", "not_findable").
It lets pricing-coverage reporting exclude models that aren't priced
per-token (image/video/audio models currently drag the "% priced" number
down even though they can never carry a per-token price), and lays the
groundwork for non-token pricing.

The default model catalog is entirely per-token, so `seed` and
`syncLlmCatalog` set `pricing_unit="tokens"` on those rows. The admin
LLM model form (create + edit) and the admin API get a pricing-unit
selector so admin-curated models can set it; existing rows can stay
unset.

Auto-discovered models get their unit from the model-registry pipeline,
which lands separately.

---------

Co-authored-by: devin-ai-integration[bot] <158243242+devin-ai-integration[bot]@users.noreply.github.com>

internal-packages/database/prisma/migrations/20260603132613_add_pricing_unit_to_llm_models/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "public"."llm_models" ADD COLUMN     "pricing_unit" TEXT;

internal-packages/database/prisma/schema.prisma

@@ -2909,6 +2909,8 @@ model LlmModel {
   capabilities    String[] @default([]) @map("capabilities")
   isHidden        Boolean  @default(false) @map("is_hidden")
   baseModelName   String?  @map("base_model_name")
+  // "tokens", "images", "characters", "minutes", "requests", "free", "not_findable"; null until researched
+  pricingUnit     String?  @map("pricing_unit")
 
   pricingTiers LlmPricingTier[]
   prices       LlmPrice[]

internal-packages/llm-model-catalog/src/seed.ts

@@ -46,6 +46,7 @@ export async function seedLlmPricing(prisma: PrismaClient): Promise<{
           capabilities: catalog?.capabilities ?? [],
           isHidden: catalog?.isHidden ?? false,
           baseModelName: catalog?.baseModelName ?? null,
+          pricingUnit: "tokens",
         },
       });
 

internal-packages/llm-model-catalog/src/sync.ts

@@ -74,6 +74,7 @@ export async function syncLlmCatalog(prisma: PrismaClient): Promise<{
             catalog?.baseModelName === undefined
               ? existing.baseModelName
               : catalog.baseModelName,
+          pricingUnit: existing.pricingUnit ?? "tokens",
         },
       });