refactor(storage): drop per-upload storage_driver, use storage_config as single source

Remove w_uploads.storage_driver and route all read/write/delete paths through
storage.Active() backed by storage_config.driver. Block direct driver switches
when uploads exist; require migration task instead. Simplify migration to
cursor-based file_path iteration without per-row driver updates.

internal/db/migrator/goose/postgres/202606180001_drop_upload_storage_driver.sql

@@ -0,0 +1,7 @@
+-- +goose Up
+DROP INDEX IF EXISTS idx_w_uploads_storage_driver_status;
+ALTER TABLE w_uploads DROP COLUMN IF EXISTS storage_driver;
+
+-- +goose Down
+ALTER TABLE w_uploads ADD COLUMN IF NOT EXISTS storage_driver VARCHAR(50) NOT NULL DEFAULT 'local';
+CREATE INDEX IF NOT EXISTS idx_w_uploads_storage_driver_status ON w_uploads (storage_driver, status);

internal/db/migrator/goose/sqlite/202606180001_drop_upload_storage_driver.sql

@@ -0,0 +1,8 @@
+-- +goose Up
+DROP INDEX IF EXISTS idx_w_uploads_storage_driver_status;
+-- SQLite lacks DROP COLUMN IF EXISTS; goose runs this only after initial schema created the column.
+ALTER TABLE w_uploads DROP COLUMN storage_driver;
+
+-- +goose Down
+ALTER TABLE w_uploads ADD COLUMN storage_driver VARCHAR(50) NOT NULL DEFAULT 'local';
+CREATE INDEX IF NOT EXISTS idx_w_uploads_storage_driver_status ON w_uploads (storage_driver, status);