feat: add DBML schema files and relspecgo migration generation

- Add schema/*.dbml covering all existing tables (001-019)
- Wire relspecgo via make generate-migrations target
- Add make check-schema-drift for CI drift detection
- Add schema/README.md documenting the DBML-first workflow

Closes #19

schema/files.dbml

@@ -0,0 +1,21 @@
+Table stored_files {
+  id bigserial [pk]
+  guid uuid [unique, not null, default: `gen_random_uuid()`]
+  thought_id uuid [ref: > thoughts.guid]
+  project_id uuid [ref: > projects.guid]
+  name text [not null]
+  media_type text [not null]
+  kind text [not null, default: 'file']
+  encoding text [not null, default: 'base64']
+  size_bytes bigint [not null]
+  sha256 text [not null]
+  content bytea [not null]
+  created_at timestamptz [not null, default: `now()`]
+  updated_at timestamptz [not null, default: `now()`]
+
+  indexes {
+    thought_id
+    project_id
+    sha256
+  }
+}