fix: make the database disk image 6 GB instead of 2 GB

- VOLUMES["db"] goes from 2048 to 6144 MiB, so every new instance gets a 6 GB database image.
  Instance 4 filled its 2 GB image with analytics logs on 2026-10-14: Postgres crashed and could
  not finish crash recovery without free space. Instance 2 had already been grown to 6 GB by hand.
- Measured on instance 2: about 1.2 GB of logs a day under browser-test traffic, kept for
  LOG_RETENTION (2 days), plus up to 1 GB of write-ahead log. That peaks near 4.5 GB.
- Only instances created after this change get the larger image; existing ones keep their size.
- README disk line, the two unit tests that assert the db volume size, and the version
  (1.6.3 -> 1.6.4 in VERSION, supaspin and README.md) are updated.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/test_supaspin.py

@@ -143,7 +143,7 @@ class Values(unittest.TestCase):
 
     def test_volume_caps(self):
         p = self.v["persistence"]
-        self.assertEqual(p["db"]["size"], "2048Mi")
+        self.assertEqual(p["db"]["size"], "6144Mi")
         self.assertEqual(p["storage"]["size"], "1024Mi")
         for vol in ("functions", "snippets", "deno", "pgsodium"):
             self.assertEqual(p[vol]["size"], "256Mi")
@@ -478,7 +478,7 @@ class Manifests(unittest.TestCase):
         pv = sp.pv_manifest(CFG, 2, "db")
         self.assertEqual(pv["spec"]["claimRef"], {"namespace": "supaspin-2", "name": "supa2-supabase-db"})
         self.assertEqual(pv["spec"]["local"]["path"], "/data/supabase/2/db/data")
-        self.assertEqual(pv["spec"]["capacity"]["storage"], "2048Mi")
+        self.assertEqual(pv["spec"]["capacity"]["storage"], "6144Mi")
         self.assertEqual(pv["spec"]["persistentVolumeReclaimPolicy"], "Retain")
         self.assertEqual(pv["spec"]["nodeAffinity"]["required"]["nodeSelectorTerms"][0]
                          ["matchExpressions"][0]["values"], ["node1"])