refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/plugins/domain/upload/migrations/00001_initial.sql

@@ -0,0 +1,49 @@
+-- +goose Up
+-- +goose StatementBegin
+CREATE TABLE IF NOT EXISTS w_upload_stats (
+    dimension VARCHAR(32) NOT NULL,
+    stat_key VARCHAR(64) NOT NULL DEFAULT '',
+    file_count BIGINT NOT NULL DEFAULT 0,
+    file_size BIGINT NOT NULL DEFAULT 0,
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (dimension, stat_key)
+);
+
+CREATE INDEX IF NOT EXISTS idx_w_uploads_status_created_at ON w_uploads (status, created_at);
+CREATE INDEX IF NOT EXISTS idx_w_uploads_hash_file_size_status ON w_uploads (hash, file_size, status);
+
+ALTER TABLE w_uploads ADD COLUMN IF NOT EXISTS access_mode INTEGER NOT NULL DEFAULT 0;
+UPDATE w_uploads SET access_mode = 1 WHERE type = 'avatar';
+
+-- Backfill upload stats
+INSERT INTO w_upload_stats (dimension, stat_key, file_count, file_size)
+SELECT 'total', '', COUNT(*), COALESCE(SUM(file_size), 0)
+FROM w_uploads
+WHERE status != 'deleted'
+ON CONFLICT (dimension, stat_key) DO UPDATE SET
+    file_count = EXCLUDED.file_count,
+    file_size = EXCLUDED.file_size,
+    updated_at = CURRENT_TIMESTAMP;
+
+INSERT INTO w_upload_stats (dimension, stat_key, file_count, file_size)
+SELECT
+    'type',
+    COALESCE(NULLIF(type, ''), 'generic'),
+    COUNT(*),
+    COALESCE(SUM(file_size), 0)
+FROM w_uploads
+WHERE status != 'deleted'
+GROUP BY COALESCE(NULLIF(type, ''), 'generic')
+ON CONFLICT (dimension, stat_key) DO UPDATE SET
+    file_count = EXCLUDED.file_count,
+    file_size = EXCLUDED.file_size,
+    updated_at = CURRENT_TIMESTAMP;
+-- +goose StatementEnd
+
+-- +goose Down
+-- +goose StatementBegin
+DROP TABLE IF EXISTS w_upload_stats;
+DROP INDEX IF EXISTS idx_w_uploads_hash_file_size_status;
+DROP INDEX IF EXISTS idx_w_uploads_status_created_at;
+ALTER TABLE w_uploads DROP COLUMN IF EXISTS access_mode;
+-- +goose StatementEnd