12-factor: FLAPI_PORT + FLAPI_HOST env vars

[jrosskopf]

Follow-up from epic #40 / PR #57. `FLAPI_CONFIG` and `FLAPI_LOG_LEVEL`
shipped in v26.05.23; `FLAPI_PORT` and `FLAPI_HOST` were explicitly
deferred to keep the 12-factor scope narrow.

Scope

Wire two more env-var fallbacks in `src/main.cpp` following the same
pattern as `FLAPI_CONFIG` / `FLAPI_LOG_LEVEL`:

• `FLAPI_PORT` falls back for `-p` / `--port`. Today port is
  CLI-flag-or-config-file only. Must coexist with the existing
  `server.http-port` config field (precedence:
  CLI > env > config > built-in default).
• `FLAPI_HOST` falls back for the bind host. There is no CLI flag
  for host today; if we want CLI > env semantics we should add a
  `--host` flag too.

Acceptance

• argparse `is_used()` precedence enforced (same idiom as Self-packaging #7: 12-factor env-var hardening (FLAPI_CONFIG + FLAPI_LOG_LEVEL) #47).
• Invalid `FLAPI_PORT` (non-int, out-of-range) → single-line stderr
  error, exit 1 (mirrors invalid `FLAPI_LOG_LEVEL` behaviour).
• 4 new cases in `test/integration/test_env_overrides.py`
  (`FLAPI_PORT` precedence, invalid `FLAPI_PORT` rejected, same for
  `FLAPI_HOST`).
• `docs/CLI_REFERENCE.md` + `docs/CONFIG_REFERENCE.md` 12-factor
  checklist + landing-page `docs/tools/server-cli.md` updated.

References

• PR feat(self-packaging #47): 12-factor env vars FLAPI_CONFIG + FLAPI_LOG_LEVEL #57 "Out of scope (deferred)" section
• v26.05.23 release notes "Out of scope" section