fix(db): rename pgSchema exports to prevent barrel collision

chat/image/mesh modules all exported a generic `const schema`
binding. When packages/db/src/schema/index.ts did `export * from
"./chat"` + `export * from "./image"` + `export * from "./mesh"`,
TypeScript's ambiguous-re-export rule silently dropped the colliding
bindings — drizzle-kit's introspection could not find the pgSchema
instances, so CREATE SCHEMA statements were never emitted. The
migration worked on the prior dev DB only because chat/image already
existed from an earlier turbostarter run; a fresh clone would fail.

pdf.ts already used `pdfSchema` (unique name). Applied the same
pattern everywhere:
- chat.ts:  `export const chatSchema = pgSchema("chat")`
- image.ts: `export const imageSchema = pgSchema("image")`
- mesh.ts:  `export const meshSchema = pgSchema("mesh")`

Also added `CREATE EXTENSION IF NOT EXISTS vector` at the top of the
migration (pgvector is used by pdf.embedding — the generated
migration assumed it was pre-enabled).

Verified end-to-end against a fresh pgvector/pgvector:pg17 container:
`pnpm drizzle-kit migrate` applies cleanly from scratch, all 7 mesh.*
tables + chat/image/pdf/mesh schemas created correctly.

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

packages/db/migrations/meta/_journal.json

@@ -5,8 +5,8 @@
     {
       "idx": 0,
       "version": "7",
-      "when": 1775335397081,
-      "tag": "0000_sloppy_stryfe",
+      "when": 1775336269295,
+      "tag": "0000_living_namora",
       "breakpoints": true
     }
   ]