Add secret value validation to reject null bytes and values exceeding 32KB.
The 32KB limit applies uniformly to both env var and file secrets because the
value field is shared and the destination can change after creation.
Add file path validation to also reject null bytes and paths exceeding 4096
bytes.
Wire up secret value validation into both POST and PATCH handlers.
Add the five REST endpoints for managing user secrets, SDK client
methods, and handler tests.
Endpoints:
- `POST /api/v2/users/{user}/secrets`
- `GET /api/v2/users/{user}/secrets`
- `GET /api/v2/users/{user}/secrets/{name}`
- `PATCH /api/v2/users/{user}/secrets/{name}`
- `DELETE /api/v2/users/{user}/secrets/{name}`
Routes are registered under the existing `/{user}` group with
`ExtractUserParam`. The delete query was changed from `:exec` to
`:execrows` so the handler can distinguish "not found" from success
(DELETE with `:exec` silently returns nil for zero affected rows).
Zach