refactor: move secid_server.py to factory pattern for testability

[kurtseifried]

Summary

Moves `secid_server.py` from "all bootstrap at module import" to the standard FastAPI factory pattern. Unblocks HTTP-layer testing and makes the module ASGI-deployable.

Pre-Phase-1 prerequisite for the broader testing-maturity initiative.

The problem this solves

Pre-refactor, importing `secid_server` triggered:

1. `argparse.parse_known_args()` — runs at import time
2. Registry-path discovery with `sys.exit(1)` if none found
3. Storage initialization
4. App and route definitions
5. Conditional MCP mount

Pytest couldn't safely `import secid_server` without either crashing (no registry directory in test environment) or loading a host-specific registry path. CI smoke tests had to skip importing the module entirely.

The fix

Standard FastAPI factory pattern:

```python
@DataClass
class ServerConfig:
registry_dirs: list[str]
storage_type: str = "memory"
storage_kwargs: dict = field(default_factory=dict)
load_mode: str = "lazy"

def create_app(config: ServerConfig) -> FastAPI:
"""Side-effect-free factory."""
...

def main(argv: Optional[list[str]] = None) -> int:
"""CLI entry point — parses args, calls create_app, runs uvicorn."""
...

if name == "main":
sys.exit(main())
```

What's now testable

Six new tests using `fastapi.testclient.TestClient`:

• `test_create_app_factory_returns_fastapi_app`
• `test_health_endpoint_returns_ok`
• `test_resolve_endpoint_returns_envelope_for_garbage_input`
• `test_resolve_endpoint_requires_secid_param`
• `test_create_app_no_side_effects_on_import`
• `test_secid_server_module_imports_without_side_effects` (regression guard for the refactor itself)

ASGI deployment path

```python

myapp.py

from secid_server import create_app, ServerConfig
import os

config = ServerConfig(
registry_dirs=os.environ.get("SECID_REGISTRY", "./registry").split(":"),
storage_type=os.environ.get("SECID_STORAGE", "memory"),
)
app = create_app(config)
```

```bash
uvicorn myapp:app --host 0.0.0.0 --port 8000
```

Verification

• 13/13 tests pass locally
• CLI verified: `python3 secid_server.py --help` produces the same usage as before
• `from secid_server import create_app, ServerConfig, main` works

CI changes

`pip install httpx` added to the test workflow (FastAPI's TestClient dependency).

🤖 Generated with Claude Code