fix(privacy): encrypt Coach conversation titles and custom-metric notes at rest

The title of a Coach conversation is the first 80 characters of the
opening message, and the note on a custom-metric reading is free text;
both were stored readable while every other note was encrypted.

Migration 0357 adds coach_conversations.title_encrypted and
custom_metric_entries.note_encrypted (the title loses its NOT NULL) plus
partial indexes on the backfill's discovery predicate. Every writer now
stores ciphertext only: conversation create, proactive nudge, reminder
surfacing, rename, entry create and edit, and both restore formats.
Readers prefer the ciphertext and fall back to the old column for rows
not yet migrated; a ciphertext that does not open throws instead of
falling back. The history title search decrypts in memory, since SQL can
no longer match the title.

free-text-encryption-backfill is a boot-time, idempotent pg-boss job
(staggered past the startup storm, daily discovery tick) that seals each
remaining readable value and nulls it in one transaction per row. A pass
that sees a row twice stops with an error instead of looping.

Both columns are registered for key rotation (registry, CLI script, and
the in-app rotation that walks the registry). A disaster-recovery backup
carries ciphertext only, sealing a not-yet-migrated row on the way out;
a portable export carries readable text. The old columns are marked for
removal in a later release.

scripts/rotate-encryption-key.ts

@@ -408,7 +408,7 @@ async function main() {
   }
 
   // ───── Coach (Bytes columns) ─────
-  // "encryptedContent" "summaryEncrypted" "factEncrypted"
+  // "encryptedContent" "summaryEncrypted" "titleEncrypted" "factEncrypted"
   results.push(
     await rotateBytesColumn(
       "CoachMessage",
@@ -423,6 +423,16 @@ async function main() {
       prisma.coachConversation,
     ),
   );
+  // v1.39.3 — the conversation title. NULL on a row the free-text backfill
+  // has not reached yet (its title is still in the readable column), which
+  // `rotateBytesColumn` skips.
+  results.push(
+    await rotateBytesColumn(
+      "CoachConversation",
+      "titleEncrypted",
+      prisma.coachConversation,
+    ),
+  );
   results.push(
     await rotateBytesColumn("CoachFact", "factEncrypted", prisma.coachFact),
   );
@@ -681,6 +691,17 @@ async function main() {
     ),
   );
 
+  // ───── v1.39.3 custom-metric reading note (Bytes column) ─────
+  // NULL on a reading without a note, and on a row the free-text backfill has
+  // not reached yet; `rotateBytesColumn` skips both.
+  results.push(
+    await rotateBytesColumn(
+      "CustomMetricEntry",
+      "noteEncrypted",
+      prisma.customMetricEntry,
+    ),
+  );
+
   // ───── v1.38.0 dose free text (Bytes column) ─────
   // The note on an administered dose — a reaction, a sore arm. Always
   // encrypted on write, so every non-null row rotates.