refactor(server): extract /api/tunings into routers/tunings.py + lib/appconfig.py (R3) (#858)

The merged-tuning-catalog route moves to lib/routers/tunings.py, verbatim except
@app->@router, CONFIG_DIR->appstate.config_dir, and the two seam substrates it
needed:

  - lib/appconfig.py — the pure config.json reader `_load_config` (used by ~11
    server sites + future config-reading routers). server.py re-imports it, so
    those call sites and any `server._load_config` test reference are unchanged.
  - appstate.tuning_providers — the TuningProviderRegistry instance injected by
    reference (a stable object mutated in place via register()/unregister()), so
    the router reads the same registry plugins populate through plugin_context.
    The instance stays defined in server.py, so `server.tuning_providers` still
    resolves — zero test retargets.

The tuning constants (DEFAULT_REFERENCE_PITCH, TUNING_PRESET_MIDIS,
freqs_to_midis) already live in lib/tunings.py and are imported directly.

server.py: 6,960 -> 6,917.

Verified: pyflakes clean (bar the pre-existing unused `tuning_name` import);
route table IDENTICAL (143); full pytest 2400 passed (110 tuning/config cases);
eslint 0. Boot smoke: /api/tunings serves referencePitch + tunings + tuningMidis.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/appconfig.py

@@ -0,0 +1,28 @@
+"""Reading the app's config.json — the one shared, pure helper (R3).
+
+Extracted verbatim from server.py so route modules that need a config value
+(reference pitch, server_config, …) can read it without reaching back into the
+host file. server.py re-imports it, so its ~11 call sites and any
+`server._load_config` test reference keep resolving unchanged.
+"""
+
+import json
+
+
+def _load_config(config_file):
+    """Read and parse config.json. Returns the parsed dict, or None if
+    the file is missing, unreadable, invalid JSON, or parses to a
+    non-dict (e.g. the file contains `[]` or `42`). Callers treat None
+    as "fall back to defaults". Shared between GET and POST so both
+    handle bad files the same way."""
+    if not config_file.exists():
+        return None
+    try:
+        # Explicit UTF-8: save_settings()/import write config.json as
+        # UTF-8 bytes, so the read must not depend on the platform's
+        # default text encoding (cp1252 on Windows would mojibake or
+        # UnicodeDecodeError on a non-ASCII DLC path).
+        parsed = json.loads(config_file.read_text(encoding="utf-8"))
+    except Exception:
+        return None
+    return parsed if isinstance(parsed, dict) else None