feat(meditate): dreams on the server, sealed and synced like the sittings

The dream journal's table (ADR 0038, D14): `dream`, in the sync engine's
database, a row a dream, its content sealed on the device (key ID,
payload, nonce), so the instance sees only that a dream exists, its size
and when it changed. The same rules as the sittings: the sync engine's
connector refuses a call without a session, each row lives in its
person's space, and deleting the person deletes their dreams with it
(the deletion test now writes and counts a dream too, and leaves
another person's).

The migration `20261103130213716-dreams`:
- `CREATE OR REPLACE FUNCTION gen_random_uuid_v7()`: the generator
  repeats the UUID v7 function every migration; replacing it with the
  same body changes nothing.
- `CREATE TABLE "dream"`: `id` a UUID v7 by default, `spaceId` the
  person's sync space, `keyId` the key it is sealed with, `payload` the
  sealed bytes (not null), `nonce` the sealing's nonce.
- `CREATE INDEX "dream_space_idx"` on `spaceId`, so deleting a space
  finds its dreams without scanning the table.
- The foreign key from `spaceId` to `offline_sync_spaces("id")` with
  `ON DELETE CASCADE`: deleting the space deletes its dreams.
- The version rows of every module in `serverpod_migrations`.
A new, empty table: nothing existing changes and no data moves.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/meditate/halkyon_meditate_client/lib/migrations/migration_registry.dart

@@ -5,6 +5,7 @@ library;
 import 'package:serverpod_database/serverpod_database.dart';
 
 part '20261103074902604-sittings/migration.dart';
+part '20261103130213716-dreams/migration.dart';
 
 /// Migration registry for the client-side database.
 class MigrationRegistry {
@@ -12,6 +13,7 @@ class MigrationRegistry {
   /// New migrations are always appended at the end.
   static final List<MigrationVersionSql> migrations = [
     _Migration20261103074902604_sittings(),
+    _Migration20261103130213716_dreams(),
   ];
 
   /// List of all client-side database migration versions.