feat(files): implement file storage functionality with save, load, and list operations

migrations/016_create_stored_files.sql

@@ -0,0 +1,20 @@
+create table if not exists stored_files (
+  id         bigserial   primary key,
+  guid       uuid        not null default gen_random_uuid(),
+  thought_id uuid        references thoughts(guid) on delete set null,
+  project_id uuid        references projects(guid) on delete set null,
+  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(),
+  constraint stored_files_guid_unique unique (guid)
+);
+
+create index if not exists stored_files_thought_id_idx on stored_files (thought_id);
+create index if not exists stored_files_project_id_idx on stored_files (project_id);
+create index if not exists stored_files_sha256_idx on stored_files (sha256);