feat(self-packaging #47): 12-factor env vars FLAPI_CONFIG + FLAPI_LOG_LEVEL

[jrosskopf]

Part of epic #40. Stacked on #56 -> #55 -> #54 -> #53 -> #52 -> #51.

Summary

A bundled flapi binary should be configurable purely via environment
variables (12-factor app). This PR closes two known gaps:

• FLAPI_CONFIG falls back for -c / --config. The CLAUDE.md docs
  already mentioned it as if it worked; this PR actually wires it.
• FLAPI_LOG_LEVEL falls back for --log-level. Invalid values cause
  flapi to exit with a clear single-line error rather than silently
  coercing -- typos like FLAPI_LOG_LEVEL=DEBUG (caps) surface
  immediately.

Precedence

CLI flag  >  env var  >  built-in default

Enforced via argparse's is_used() query -- we only consult getenv
when the user didn't pass the flag.

Implementation (src/main.cpp, 24 lines)

if (!program.is_used(\"--config\")) {
    if (const char* env = std::getenv(\"FLAPI_CONFIG\"); env && *env) {
        config_file = env;
    }
}
if (!program.is_used(\"--log-level\")) {
    if (const char* env = std::getenv(\"FLAPI_LOG_LEVEL\"); env && *env) {
        log_level = env;
    }
}
if (log_level != \"debug\" && log_level != \"info\" &&
    log_level != \"warning\" && log_level != \"error\") {
    std::cerr << \"flapi: invalid log level '\" << log_level
              << \"'; must be one of: debug, info, warning, error\\n\";
    return 1;
}

Integration tests (test/integration/test_env_overrides.py, 6 cases, ~4 sec)

#	Test	What it asserts
1	invalid FLAPI_LOG_LEVEL	exit 1; error names the offending value
2	FLAPI_LOG_LEVEL=debug	DEBUG log lines appear at validate-config
3	CLI --log-level error + env debug	no DEBUG lines (CLI wins)
4	FLAPI_CONFIG=<path> no -c	env path is loaded
5	-c <valid> + FLAPI_CONFIG=<broken>	CLI wins; validate succeeds
6	neither flag nor env, default cwd	flapi.yaml in cwd is loaded

============================== 6 passed in 4.25s ===============================

Docs

• docs/CLI_REFERENCE.md: env-var rows + precedence lists under
  --config and --log-level, plus example shell invocations.
• docs/CONFIG_REFERENCE.md: new "12-factor checklist" subsection
  consolidating every env var flapi reads (startup vs query time),
  with a pointer to the flapi pack secret deny list as the
  packaging-time enforcement of "secrets from env, never from bundle".

Out of scope (deferred -- user explicitly narrowed #47 to these two vars)

• FLAPI_PORT / FLAPI_HOST
• Full credential env-var audit / inventory

Test plan

• 6 new integration tests pass on Linux x86_64 debug.
• Existing 8 self-packaging tests still pass (the precedence
  logic is in a separate code path from pack / info / unpack).
• Smoke: FLAPI_LOG_LEVEL=verbose flapi --validate-config -c examples/flapi.yaml exits 1 with the documented error message.
• CI cross-platform once the whole stack lands.

Closes #47. Part of #40.