feat(16-01): migrate schema to pgTable and add users table with userId columns

- Rewrite schema.ts from sqlite-core to pg-core (pgTable, serial, timestamp, doublePrecision)
- Add users table with id, logtoSub (unique), createdAt
- Add userId FK column to items, categories, threads, setups, apiKeys, oauthTokens
- Add composite unique constraint on categories(userId, name)
- Change settings PK to composite (userId, key)
- Remove global Uncategorized seed from seed.ts (now per-user lazy)
- Generate Drizzle pg migration

drizzle-pg/meta/_journal.json

@@ -0,0 +1,13 @@
+{
+  "version": "7",
+  "dialect": "postgresql",
+  "entries": [
+    {
+      "idx": 0,
+      "version": "7",
+      "when": 1775377947759,
+      "tag": "0000_thankful_loners",
+      "breakpoints": true
+    }
+  ]
+}