feat: closed-set admin knobs are pickers, not free text

log_level, search_source, download_layout and the transcode hw/preset
were typed by hand, and every consumer carried its own guess at what a
typo meant. The value set is now a Literal in config.py: pydantic
rejects a bad env value at boot (any letter case accepted), sysconfig
refuses a bad override and derives the admin picker's options from the
same Literal, and the consumers read the value as-is.

The Configuration page renders a choice key as a segmented bar, or a
dropdown past six options, with translated option names. Its tabs now
follow the registry's order instead of a hand-kept copy that had
already dropped the explore group.

A single-choice FilterSelect now opens on the option in use: in the
browser it could land on the first row instead.

backend/tests/test_sysconfig_registry.py

@@ -0,0 +1,93 @@
+"""The sysconfig registry: its order is the admin page's tab order, and a closed-set knob is a CHOICE.
+
+The admin Configuration page used to keep its own hand-written group order, which had already drifted
+(`explore` was missing, so its tab sorted last). The page now follows SPECS, so SPECS has to read in
+tab order: one run of keys per group.
+
+A closed-set admin knob is a CHOICE, not free text (E22 S2 follow-up).
+
+`log_level`, `search_source`, `download_layout` and the transcode `hw`/`preset` used to be free-text
+fields: the admin could type anything and every consumer carried its own `or "auto").lower()` guess
+at what a typo meant. Now the value set is a `Literal` in config.py, pydantic holds the env to it at
+boot, sysconfig holds a stored override to it, and the admin page renders a picker from the same list.
+The DB lane (`db` fixture) covers the store; the rest is pure logic.
+"""
+
+import pytest
+from pydantic import ValidationError
+
+from app import sysconfig
+from app.config import Settings, choices_of
+
+CHOICE_KEYS = {s.key for s in sysconfig.SPECS if s.type == "choice"}
+
+
+def test_each_group_is_one_run_of_keys_so_the_registry_reads_in_tab_order():
+    runs = [
+        s.group
+        for i, s in enumerate(sysconfig.SPECS)
+        if i == 0 or sysconfig.SPECS[i - 1].group != s.group
+    ]
+    assert len(runs) == len(set(runs)), runs
+    assert runs[:2] == ["access", "email"]
+
+
+def test_every_closed_set_knob_is_a_choice_whose_options_come_from_config():
+    assert CHOICE_KEYS == {
+        "search_source",
+        "download_layout",
+        "log_level",
+        "plex_transcode_hw",
+        "plex_transcode_preset",
+    }
+    for s in sysconfig.SPECS:
+        field = Settings.model_fields[s.default_attr]
+        if s.type == "choice":
+            assert s.choices, s.key
+            assert field.default in s.choices, s.key
+        else:
+            # A Literal-typed setting exposed as "str" would be a free-text field again.
+            assert s.choices == () and choices_of(field.annotation) == (), s.key
+
+
+def test_the_env_accepts_any_letter_case_and_rejects_an_unknown_value():
+    assert Settings(log_level="debug").log_level == "DEBUG"
+    assert Settings(search_source=" API ").search_source == "api"
+    with pytest.raises(ValidationError):
+        Settings(log_level="verbose")
+
+
+class _Row:
+    def __init__(self, value):
+        self.value, self.encrypted = value, False
+
+
+class _FakeDb:
+    """Just enough of a Session for sysconfig's reads: one stored override row per key."""
+
+    def __init__(self, **rows):
+        self.rows = {k: _Row(v) for k, v in rows.items()}
+
+    def get(self, _model, key):
+        return self.rows.get(key)
+
+
+def test_a_stored_value_reads_back_canonical_and_a_stale_one_falls_back_to_the_default():
+    assert sysconfig.get_str(_FakeDb(plex_transcode_hw="OFF"), "plex_transcode_hw") == "off"
+    # A value that is no longer (or never was) a choice is not passed through to the consumer.
+    assert sysconfig.get_str(_FakeDb(plex_transcode_hw="maybe"), "plex_transcode_hw") == "auto"
+
+
+def test_describe_hands_the_picker_its_options():
+    items = {i["key"]: i for g in sysconfig.describe(_FakeDb())["groups"].values() for i in g}
+    assert items["log_level"]["choices"] == ["TRACE", "DEBUG", "INFO", "WARNING", "ERROR"]
+    assert items["log_level"]["value"] == "INFO"
+    assert items["quota_daily_budget"]["choices"] == []
+
+
+def test_set_value_stores_the_canonical_spelling_and_refuses_anything_else(db):
+    sysconfig.set_value(db, "search_source", "API")
+    assert sysconfig.get_str(db, "search_source") == "api"
+    with pytest.raises(ValueError, match="scrape, api"):
+        sysconfig.set_value(db, "search_source", "bing")
+    db.rollback()