refactor(server): drop stale prompt text, duplicated routes and dead options

- lib/system-prompts.ts: delete the extended prompt kept only to reach
  the 4096-token cache threshold of two old Claude models; describe the
  current canvas-first UI in the App Features section
- app/api/chat/route.ts: remove the server-side example cache (the
  browser answers examples itself), the ENABLE_HISTORY_XML_REPLACE
  option and its helper, the unread messageMetadata fields; the
  DEBUG_LLM_PAYLOAD blocks become one describeMessages summary
- app/api/log-feedback and log-save share scoreLatestTrace in
  lib/langfuse.ts
- app/api/admin/test-model and validate-model call a shared
  testModelConnection() instead of one route faking a request to the
  other and passing trust through the body
- lib/dynamo-quota-manager.ts reads the failing item from the
  conditional update instead of a second GetItem
- lib/ai-providers.ts: geminiThinkingOptions helper for the google and
  vertexai cases, drop the unused fourth resolveBaseURL parameter
- edge-functions: always stream, shrink messageSchema, drop model
  aliases nothing sends
- delete app/api/chat/xml_guide.md, parseOutputTokenLimit, unused type
  exports and export keywords on file-internal symbols
- env.example: drop variables nothing reads, document the quota,
  validation and analytics variables, mention the apiKeyEnv array form
  (also in docs/*/ai-providers.md)

lib/server-model-config.ts

@@ -10,7 +10,7 @@ export const ProviderNameSchema: z.ZodType<ProviderName> = z
         message: "Invalid provider name",
     })
 
-export const ServerProviderSchema = z.object({
+const ServerProviderSchema = z.object({
     name: z.string().min(1),
     provider: ProviderNameSchema,
     models: z.array(z.string().min(1)),
@@ -30,7 +30,6 @@ export const ServerModelsConfigSchema = z.object({
     providers: z.array(ServerProviderSchema),
 })
 
-export type ServerProviderConfig = z.infer<typeof ServerProviderSchema>
 export type ServerModelsConfig = z.infer<typeof ServerModelsConfigSchema>
 
 export interface FlattenedServerModel {