feat: force a seller scan over active conversations from the inbox

Adds a "Scan for sellers" button to the conversations header, backed by
POST /api/conversations/seller-scan.

The sweep reuses the existing seller detection rather than inventing a
second one: the configured keyword/domain list, the seller-link funnel
patterns, and the aiReplyScreening confidences all decide the verdict, so
a swept conversation lands in the state the live reply loop would have
given it. Being operator-forced, it deliberately ignores filterSellers
and aiReplyScreening.enabled.

AI is the last resort. Conversations are skipped when already flagged or
killed, settled for free by the deterministic rules, or skipped against a
durable sellerScreenedKey watermark when nothing new has arrived since
the last clean sweep. Only the remainder reaches the model, batched eight
at a time through a new screenConversationsForSellers, so the system
prompt and round trip are paid per batch instead of per conversation. A
classifier outage degrades the sweep instead of failing it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/api-contracts/src/chats.ts

@@ -163,6 +163,32 @@ export const BulkMarkConversationsReadResponseSchema = z.strictObject({
   updated: z.number().int().nonnegative(),
   failed: z.array(z.string()),
 });
+/** One conversation an operator-forced seller sweep decided to flag. */
+export const SellerScanFlagSchema = z.strictObject({
+  conversationId: z.string(),
+  username: z.string(),
+  /** Which rule settled it. Everything but `ai` costs no tokens. */
+  source: z.enum(['keyword', 'domain', 'funnel', 'ai']),
+  reason: z.string(),
+  /** Present for AI verdicts only. */
+  confidence: z.number().min(0).max(1).optional(),
+  /** True when the sweep also ended the conversation, per the kill policy. */
+  killed: z.boolean(),
+});
+export const SellerScanResponseSchema = z.strictObject({
+  success: z.literal(true),
+  scanned: z.number().int().nonnegative(),
+  flagged: z.array(SellerScanFlagSchema),
+  alreadyFlagged: z.number().int().nonnegative(),
+  unchanged: z.number().int().nonnegative(),
+  deterministic: z.number().int().nonnegative(),
+  aiScreened: z.number().int().nonnegative(),
+  aiRequests: z.number().int().nonnegative(),
+  budgetSkipped: z.number().int().nonnegative(),
+  emptySkipped: z.number().int().nonnegative(),
+  /** True when a classifier request failed and part of the sweep was skipped. */
+  degraded: z.boolean(),
+});
 export const StartConversationRequestSchema = z.strictObject({
   accountId: z.string().min(1),
   username: z.string().min(1),
@@ -188,6 +214,8 @@ export type ConversationTriageStatus = z.infer<typeof ConversationTriageStatusSc
 export type ConversationAutomationStatus = z.infer<typeof ConversationAutomationStatusSchema>;
 export type ConversationPolicyMode = z.infer<typeof ConversationPolicyModeSchema>;
 export type BulkMarkConversationsReadResponse = z.infer<typeof BulkMarkConversationsReadResponseSchema>;
+export type SellerScanFlag = z.infer<typeof SellerScanFlagSchema>;
+export type SellerScanResponse = z.infer<typeof SellerScanResponseSchema>;
 export type ChatSyncStatus = z.infer<typeof ChatSyncStatusSchema>;
 export type ConversationDetailResponse = z.infer<typeof ConversationDetailResponseSchema>;
 export type ConversationMediaShareOptions = z.infer<typeof ConversationMediaShareOptionsResponseSchema>;