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_server/lib/src/journal/dream.spy.yaml

@@ -0,0 +1,20 @@
+### A dream the person wrote down (ADR 0038, D14): the night it belongs to,
+### its title and text, its marks (lucid, nightmare, recurring), its clarity,
+### the mood on waking, its labels and when it was written, all in its sealed
+### payload; the instance sees that it exists, its size and when it changed.
+### Changed and deleted by the person alone. Synced.
+class: Dream
+table: dream
+database: sync
+fields:
+  ### The key it is sealed with (ADR 0002's envelope); never null in Meditate,
+  ### where everything is sealed.
+  keyId: String?
+  ### The content, sealed: UTF-8 JSON with a `schemaVersion` once opened.
+  payload: ByteData
+  ### The sealing's nonce (ADR 0002's envelope).
+  nonce: ByteData?
+indexes:
+  # Deleting a space finds its rows through this index (see content_key).
+  dream_space_idx:
+    fields: spaceId