feat(db): owner_secret_key + root_key columns on mesh for server-side signing

Completes the server-side invite-signing story. The web UI's
create-invite flow needs the mesh owner's ed25519 SECRET key to sign
each invite payload; these columns let the backend hold + use them
per mesh.

- mesh.mesh.owner_secret_key (text, nullable): ed25519 secret key
  (hex, 64 bytes) paired with owner_pubkey. Stored PLAINTEXT AT REST
  for v0.1.0. Acceptable trade-off for a managed-broker SaaS launch —
  the operator controls the key anyway. v0.2.0 will either encrypt
  with a column-level KEK or migrate to client-held keys.
- mesh.mesh.root_key (text, nullable): 32-byte shared key
  (base64url, no padding) used by channel/broadcast encryption in
  later steps. Embedded in every invite so joiners receive it at
  join time.

migrations/0002_vengeful_enchantress.sql — two ALTER TABLE ADD
COLUMN. Nullable so existing rows don't need backfill to migrate;
the backfill script populates them idempotently.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/db/migrations/0002_vengeful_enchantress.sql

@@ -0,0 +1,2 @@
+ALTER TABLE "mesh"."mesh" ADD COLUMN "owner_secret_key" text;--> statement-breakpoint
+ALTER TABLE "mesh"."mesh" ADD COLUMN "root_key" text;

packages/db/migrations/meta/_journal.json

@@ -15,6 +15,13 @@
       "when": 1775339743477,
       "tag": "0001_demonic_karnak",
       "breakpoints": true
+    },
+    {
+      "idx": 2,
+      "version": "7",
+      "when": 1775340519054,
+      "tag": "0002_vengeful_enchantress",
+      "breakpoints": true
     }
   ]
 }