chore(lint): fix 288 ruff errors + a hidden UnboundLocalError - #9

The backend could not start at all. Fixes four defects that each block import
or execution:

1. backend/config.py - Settings no longer defined OPENAI_API_KEY / OPENAI_MODEL,
   but the validation block read settings.OPENAI_API_KEY, raising
   AttributeError: 'Settings' object has no attribute 'OPENAI_API_KEY' at import.
   Regression introduced by 5e43b24, which deleted the two field definitions and
   in the same commit added the validation that reads them. Restored both fields.

2. backend/routers/chat.py:18 - docstring indented 8 spaces inside a 4-space
   function body -> IndentationError, blocking main.py.

3. scripts/cli.py:151 - string literal split across real newlines ->
   SyntaxError: unterminated string literal. Restored the \n escapes.

4. scripts/nethermind.py - main() read global API_BASE (line 251) before its
   global declaration (line 329) -> SyntaxError. Moved the declaration to the
   top of main().

Verified: import main -> OK with 67 routes; uvicorn serves / , /api/switches/,
/api/dashboard/stats, /docs, /openapi.json all 200; scripts/nethermind.py --help
runs; full tree compiles with 0 syntax errors.

The repo declared pytest config (testpaths=["tests"], python_files=["test_*.py"])
but shipped no tests — tests/ contained only a mock_switch.py helper.

- tests/conftest.py — puts backend/ on sys.path and redirects DATABASE_URL to a
  temporary sqlite file *before* main is imported, so the real switches.db is
  never touched. Tests use a per-test TestClient fixture.
- tests/test_api.py — 11 tests: app metadata, OpenAPI route presence, dashboard
  stats shape, switch list/create/read-back, dashboard count reflects a created
  switch, and 5 pure-logic unit tests of the ArubaOS/ProCurve running-config
  parser (scalars, role detection, services, VLAN block parsing, empty input).
- frontend/package-lock.json — npm synced the stale package name "nethermind"
  to "netcore" while building; matches package.json.

Verified: cd . && .venv/bin/python -m pytest tests/ -q -> 11 passed.
DB isolation verified by mtime (real switches.db untouched). ruff clean on both
new files. gitleaks clean.

Repo configured ruff (E,F,W,I,UP,B,SIM) but failed it 288 times, so lint was
noise nobody could act on. Now: ruff check . -> All checks passed.

Highlights:
- F823 (REAL BUG) backend/services/netmiko_client.py: a redundant local
  'import re' inside pull_running_config made 're' function-local, so the
  re.search() call above it raised UnboundLocalError. Removed the local import.
- E741 x5, F841 x3, SIM105 x8, SIM108 x2, E701 x2, B007, F401, F404, I001,
  UP006/035/037/041/045, F541, E402, B024: applied safe autofixes and hand
  edits. B008 (47x) is added to ignore: FastAPI requires Depends()/File()/
  Query() in argument defaults -- that is the documented DI idiom, not a bug.
- security_auditor._check_aaa_config: extracted the tangled inline conditional
  into a named variable. Behaviour verified unchanged by direct test:
  reports the missing-local-fallback finding only when 'aaa authentication
  login' is present without 'local' (1 for the bad case, 0 for the good case).
- B024: SerialConnection is a runtime-selected marker base, so an explicit
  noqa with rationale rather than adding a wrong @AbstractMethod.

Verified: ruff clean; 11 tests pass; import main -> 67 routes; whole tree
compiles; gitleaks clean.