Document svc.sh comprehensively and layer its help system

Why:
svc.sh grew into a three-kind service manager with database operations,
but its only documentation was a flat --help wall of text; the user asked
for a complete usage document and a better help experience.

Evidence:
- docs/svc.md: 14-section manual (prerequisites, quick start, concepts
  and directory layout, per-command reference for service/global/db
  commands, deploy sources, database operations handbook, configuration
  layering, registry fields and event types, safety boundaries, exit
  codes, troubleshooting table, recipe index) — every claim cross-checked
  against the implementation from the previous two rounds.
- Help layering: no-args and -h print a one-screen overview; --help
  renders the header comment reorganized into 7 labeled sections; the new
  "help [topic]" command deep-dives eight topics (model, deploy,
  lifecycle, db, dbops, config, examples, safety); usage errors now show
  the short overview plus a pointer to --help / help / docs/svc.md.
  All topics verified to render; unknown topic exits 2.
- No runtime behavior changed: end-to-end db lifecycle regression
  (create → start → backup → status → stop → rm --purge) passes after the
  refactor.
- Makefile help and README's documentation section point to docs/svc.md.

Impact:
Documentation-only plus display-layer changes; command semantics,
exit codes and file formats are untouched. Maintenance note: behavior
changes must now update both --help and docs/svc.md.

Refs: .task/feature-svc-docs-help.md（任务卡=PR 描述）

README.md

@@ -37,3 +37,4 @@ make dev      # 开发模式说明（前后端分离热更新）
 ## 文档
 
 设计方案：`modules/documents/docs/proposals/2026-09-29-system-skeleton-design-and-plan.md`（含 D1–D16 决策记录）。
+本机服务管理（后端/前端/数据库部署、生命周期、备份克隆）：`docs/svc.md`（`scripts/svc.sh`）。