feat(documents): add document_id chat discriminator + verbatim index column (mig 0230)

prisma/migrations/0230_document_chat/migration.sql

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+-- Document vault P4: chat about a document.
+--
+-- Two additive, nullable columns — no backfill, applies clean on a 0229
+-- database and on a fresh database:
+--
+--   1. `coach_conversations.document_id` — the discriminator that turns a Coach
+--      conversation into a DOCUMENT chat. NULL = a normal Coach conversation
+--      (health-record surface). Non-NULL = a chat scoped to exactly one stored
+--      document, whose only context is that document's text (no health snapshot,
+--      no tools). FK cascades: deleting the document removes its chats. A
+--      composite index covers both the Coach rail scan (`document_id IS NULL`,
+--      newest-first) and the document sheet scan (`document_id = <id>`,
+--      newest-first).
+--
+--   2. `document_content_index.verbatim_text_encrypted` — AES-256-GCM ciphertext
+--      of the VERBATIM extracted text (byte-capped, not lowercased/de-accented),
+--      stored additionally alongside the normalised `text_encrypted` so a chat
+--      can cite the document faithfully. Nullable: rows indexed before P4 carry
+--      NULL until re-indexed, and the chat route falls back to `text_encrypted`.
+
+-- AlterTable
+ALTER TABLE "coach_conversations"
+  ADD COLUMN "document_id" TEXT;
+
+-- AlterTable
+ALTER TABLE "document_content_index"
+  ADD COLUMN "verbatim_text_encrypted" BYTEA;
+
+-- CreateIndex — covers the Coach rail (document_id IS NULL) and the document
+-- sheet (document_id = <id>) newest-first scans with one composite.
+CREATE INDEX "coach_conversations_user_id_document_id_updated_at_idx"
+  ON "coach_conversations" ("user_id", "document_id", "updated_at");
+
+-- AddForeignKey — a deleted document takes its chats with it.
+ALTER TABLE "coach_conversations"
+  ADD CONSTRAINT "coach_conversations_document_id_fkey"
+  FOREIGN KEY ("document_id") REFERENCES "inbound_documents" ("id")
+  ON DELETE CASCADE ON UPDATE CASCADE;