Publish milestones per channel and add Instagram

Consent is bound to the channel it was given for: approving a post on X says
nothing about Instagram. Publishing state moves from the SocialMilestone row
into SocialMilestonePost, one row per channel, where a missing row means no
consent. The dialog asks per channel, shows the text each one will publish and
keeps a separate handle for each; Instagram captions end in hashtags because a
link there is not clickable.

Also fixes three problems in the existing X path:

- A QR code already past several thresholds produced one prompt per threshold,
  and since the post quotes the current scan count, every one of them would
  have published the same number. Only the highest threshold is announced now.
- Detection ran after every unique scan and re-read the QR code's full scan
  history just to hit skipDuplicates. Known milestones are filtered first.
- A failed post stayed failed forever because the consent dialog only opens
  once. The queue now retries three times on its own, spaces first attempts by
  SOCIAL_MILESTONE_MIN_GAP_HOURS, and Settings lists every milestone per
  channel with restart and revoke.

The worker no longer renders the card itself; it downloads the image the app
renders at /s/m/<token>/og, which also serves the new square and portrait
formats. Instagram publishing stays off until SOCIAL_MILESTONE_CHANNELS and
SOCIAL_WORKER_CHANNELS both name it.

Schema changes are manual SQL, see sql/2026-08-16_*.sql. Run both before
deploying this version.

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

prisma/schema.prisma

@@ -94,6 +94,7 @@ model User {
   // Social-success sharing preferences. A post is still never published
   // without a per-milestone approval stored below.
   xHandle             String?
+  instagramHandle     String?
   socialPromptOptOut  Boolean @default(false)
 }
 
@@ -183,14 +184,42 @@ model SocialMilestone {
   selfSharedAt DateTime?
   shareToken String? @unique
   publicShareApprovedAt DateTime?
+  attempts Int @default(0)
+  nextAttemptAt DateTime?
 
   qr   QRCode @relation(fields: [qrId], references: [id], onDelete: Cascade)
   user User   @relation(fields: [userId], references: [id], onDelete: Cascade)
+  posts SocialMilestonePost[]
 
   @@unique([qrId, kind])
   @@index([status, respondedAt])
   @@index([status, claimedAt])
   @@index([userId, status])
+  @@index([brandStatus, brandApprovedAt])
+}
+
+model SocialMilestonePost {
+  id          String   @id @default(cuid())
+  milestoneId String
+  /// "x" | "instagram". Consent is bound to the channel it was given for.
+  channel     String
+  /// approved | processing | posted | failed | revoked
+  status      String   @default("approved")
+  /// The exact text the customer read before consenting.
+  consentText String
+  handle      String?
+  approvedAt  DateTime @default(now())
+  claimedAt   DateTime?
+  postedAt    DateTime?
+  postUrl     String?
+  error       String?
+  attempts    Int      @default(0)
+  nextAttemptAt DateTime?
+
+  milestone SocialMilestone @relation(fields: [milestoneId], references: [id], onDelete: Cascade)
+
+  @@unique([milestoneId, channel])
+  @@index([channel, status, approvedAt])
 }
 
 enum QRType {