fix(saas): P2 code quality fixes + config PATCH/PUT alignment

P2 code quality (SEC2-P2-01~10):
- P2-04: Replace vague TODO with detailed Phase 2 design note in generate_embedding.rs
- P2-05: Add NOTE(fire-and-forget) annotations to 4 long-running tokio::spawn in main.rs
- P2-07: Add DESIGN NOTE to scheduler explaining sequential execution rationale
- P2-08: Add compile-time table name whitelist + runtime char validation in db.rs
- P2-02: Verified N/A (only zclaw-pipeline uses serde_yaml_bw, no inconsistency)
- P2-06: Verified N/A (bind loop correctly matches 6-column placeholders)
- P2-03: Remains OPEN (requires upstream sqlx release)

Config HTTP method alignment (B3-4):
- Fix admin-v2 config.ts: request.patch -> request.put to match backend .put() route
- Fix backend handler doc comment: PATCH -> PUT
- Add @reserved annotations to 6 config handlers without frontend callers

admin-v2/src/services/config.ts

@@ -7,5 +7,5 @@ export const configService = {
       .then((r) => r.data.items),
 
   update: (id: string, data: { value: string | number | boolean }, signal?: AbortSignal) =>
-    request.patch<ConfigItem>(`/config/items/${id}`, data, withSignal({}, signal)).then((r) => r.data),
+    request.put<ConfigItem>(`/config/items/${id}`, data, withSignal({}, signal)).then((r) => r.data),
 }