feat(coach): scaffold conversation-summary + facts storage

Extract the AES-256-GCM Bytes codec from persistence into a shared
bytes-codec module so the conversation summary and the new personal-fact
store reuse one ArrayBuffer-backed implementation. Add the CoachFact
table (migration 0116) for durable, user-scoped facts the Coach extracts
from conversations — encrypted fact text, plain category/confidence/
source for rank-without-decrypt, soft-delete for a reversible forget.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/ai/coach/bytes-codec.ts

@@ -0,0 +1,32 @@
+/**
+ * Shared AES-256-GCM ↔ `Bytes` codec for the Coach's encrypted columns.
+ *
+ * `coach_messages.encrypted_content`, `coach_conversations.summary_encrypted`,
+ * and `coach_facts.fact_encrypted` all store the same payload shape: the
+ * `encrypt()` string format from `@/lib/crypto` (`"<keyId>.<base64>"`) encoded
+ * as UTF-8 bytes. Centralised here so there is one ArrayBuffer-backed
+ * implementation every Coach persistence path shares.
+ *
+ * Prisma's `Bytes` type maps to `Uint8Array<ArrayBuffer>`, not Node's
+ * `Buffer<ArrayBufferLike>`, so we allocate a fresh ArrayBuffer-backed
+ * `Uint8Array` for writes to keep the structural type stable across Node
+ * versions.
+ */
+import { Buffer } from "node:buffer";
+
+import { decrypt, encrypt } from "@/lib/crypto";
+
+/** Encrypt a UTF-8 string into the `Bytes` payload the schema stores. */
+export function encryptToBytes(plaintext: string): Uint8Array<ArrayBuffer> {
+  const ciphertext = encrypt(plaintext);
+  const encoded = Buffer.from(ciphertext, "utf8");
+  const out = new Uint8Array(new ArrayBuffer(encoded.byteLength));
+  out.set(encoded);
+  return out;
+}
+
+/** Decrypt a `Bytes` payload back to its plaintext. Throws on a bad key id. */
+export function decryptFromBytes(buf: Uint8Array): string {
+  const text = Buffer.from(buf).toString("utf8");
+  return decrypt(text);
+}

src/lib/ai/coach/persistence.ts

@@ -10,10 +10,8 @@
  * provenance (window names, metric tags, sample counts) and never raw
  * values, so it can be queried without decryption for analytics.
  */
-import { Buffer } from "node:buffer";
-
 import { prisma } from "@/lib/db";
-import { encrypt, decrypt } from "@/lib/crypto";
+import { decryptFromBytes, encryptToBytes } from "./bytes-codec";
 
 import type {
   CoachConversationDTO,
@@ -49,27 +47,6 @@ export function summariseTitle(input: string): string {
   return `${cut.trimEnd()}…`;
 }
 
-/**
- * Encode a UTF-8 string as the AES-256-GCM payload format the schema
- * stores in `coach_messages.encrypted_content`.
- *
- * Prisma's `Bytes` type maps to `Uint8Array<ArrayBuffer>`, not Node's
- * `Buffer<ArrayBufferLike>`. We allocate a fresh ArrayBuffer-backed
- * Uint8Array so the structural type matches across Node versions.
- */
-function encryptToBytes(plaintext: string): Uint8Array<ArrayBuffer> {
-  const ciphertext = encrypt(plaintext);
-  const encoded = Buffer.from(ciphertext, "utf8");
-  const out = new Uint8Array(new ArrayBuffer(encoded.byteLength));
-  out.set(encoded);
-  return out;
-}
-
-function decryptFromBytes(buf: Uint8Array): string {
-  const text = Buffer.from(buf).toString("utf8");
-  return decrypt(text);
-}
-
 function provenanceToJson(provenance: CoachProvenance | null): string | null {
   if (!provenance) return null;
   return JSON.stringify(provenance);