feat: add initial database schema with migrations

Introduce foundational database schema with tables for `users`, `categories`, `contents`, `tags`, and `favorites`. Add foreign key relationships, constraints, and indexes for efficient querying.

backend/.migrations/meta/_journal.json

@@ -0,0 +1,20 @@
+{
+  "version": "7",
+  "dialect": "postgresql",
+  "entries": [
+    {
+      "idx": 0,
+      "version": "7",
+      "when": 1767618753676,
+      "tag": "0000_right_sally_floyd",
+      "breakpoints": true
+    },
+    {
+      "idx": 1,
+      "version": "7",
+      "when": 1768392191169,
+      "tag": "0001_purple_goliath",
+      "breakpoints": true
+    }
+  ]
+}