fix(upload): restrict cross-user private file access (UPLOAD-1)

- Add access_mode column to w_uploads table (0 = private, 1 = public) and initialize data in a single migration script
- Enforce strict ownership check for private files during download
- Allow public files to follow whitelisted public-access rules
- Default access_mode to public for avatars and private for generic uploads
- Update frontend service to support optional accessMode parameter

internal/db/migrator/goose/postgres/202606130001_add_upload_access_mode.sql

@@ -0,0 +1,6 @@
+-- +goose Up
+ALTER TABLE w_uploads ADD COLUMN access_mode INTEGER NOT NULL DEFAULT 0;
+UPDATE w_uploads SET access_mode = 1 WHERE type = 'avatar';
+
+-- +goose Down
+ALTER TABLE w_uploads DROP COLUMN access_mode;

internal/db/migrator/goose/sqlite/202606130001_add_upload_access_mode.sql

@@ -0,0 +1,6 @@
+-- +goose Up
+ALTER TABLE w_uploads ADD COLUMN access_mode INTEGER NOT NULL DEFAULT 0;
+UPDATE w_uploads SET access_mode = 1 WHERE type = 'avatar';
+
+-- +goose Down
+ALTER TABLE w_uploads DROP COLUMN access_mode;