feat(saved-searches): back saved searches with NetBox native SavedFilter

[thomaschristory]

Closes #129.

What

Saved searches are now backed by NetBox's native saved filters
(extras.saved-filters) instead of a local-only config.yaml store, making them
interchangeable with the web UI: a search saved in nsc (TUI Ctrl+W, or
applied via --saved) shows up in the NetBox web UI's filter dropdown for that
object — and a filter created in the web UI loads in the TUI (Ctrl+O).

How

• New nsc/savedfilters/ package (framework-free):
  • objecttypes.py — resolves a resource's list path to its app_label.model
    object type using NetBox's own registry (/api/core/object-types/,
    rest_api_endpoint). This is exact, not a depluralization guess (devices'
    schema is DeviceWithConfigContext, the URL is ip-addresses → ipaddress,
    etc.), and the match tolerates sub-path installs, absolute URLs, and casing.
  • params.py — translates between nsc's flat dict[str, str] and NetBox's
    list-valued parameters, plus slug generation.
  • store.py — NativeSavedFilterStore: CRUD on extras.saved-filters
    (object_types plural on writes, object_type singular filter on reads),
    upsert-by-name (create, else PATCH preserving an existing slug), with a
    config.yaml fallback.
• config.yaml fallback (ConfigFileSavedSearchStore): when NetBox is
  unreachable or the user lacks permission on extras.savedfilter, saves/loads
  degrade to the previous local store and surface a notice rather than
  failing silently.
• Wiring: NscTuiApp's three saved-search methods route through the store;
  the tui command and the --saved CLI option construct it. TUI store calls
  run off the Textual event loop (threaded, like ListScreen) so the UI never
  freezes.

Notes / decisions

• New filters are created shared=True, enabled=True so they're visible in the
  web UI like a normal saved filter (a private filter without a resolvable owner
  can be invisible).
• nsc's filter state is one value per key, so a web-UI multi-select reads as its
  first value; an explicit re-save overwrites with the current state (documented
  in params.py).
• Targets NetBox 4.x (bundled schemas 4.5/4.6); on older servers without
  /api/core/object-types/, resolution returns None and the local fallback
  is used.

Testing

• New unit tests: tests/savedfilters/ (params, object-type resolution incl.
  sub-path/casing, store CRUD + upsert + API-error fallback + duplicate-name
  handling), tests/config/test_saved_search_fallback.py,
  tests/tui/test_saved_filter_integration.py.
• Updated tests/cli/test_registration.py, tests/cli/test_tui_command.py,
  tests/tui/test_filter_screen.py for the new backing + threaded workers.
• Full suite green (1279 passed, 1 skipped); ruff + mypy --strict clean.
• Hardened against findings from an adversarial multi-agent review (sub-path
  endpoint mismatch, event-loop blocking, duplicate-name silent drop, fallback
  test gaps).

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