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/app/config.py

@@ -1,6 +1,7 @@
 from pathlib import Path
+from typing import Literal, get_args, get_origin
 
-from pydantic import model_validator
+from pydantic import ValidationInfo, field_validator, model_validator
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 # Shipped placeholder; production must override it. Used by the startup guard below.
@@ -20,6 +21,25 @@ def _version_from_file() -> str:
     return "dev"
 
 
+# Closed value sets. A setting typed with one of these is a CHOICE: pydantic rejects anything else at
+# boot, and sysconfig derives the admin picker's options from the same Literal — so the list lives in
+# exactly one place and a consumer never has to guess what a typo should fall back to.
+SearchSource = Literal["scrape", "api"]
+DownloadLayout = Literal["plex", "flat"]
+LogLevel = Literal["TRACE", "DEBUG", "INFO", "WARNING", "ERROR"]
+LogFormat = Literal["json", "text"]
+TranscodeHw = Literal["auto", "on", "off"]
+# libx264's presets, fastest first (`placebo` left out on purpose: no measurable gain for its cost).
+TranscodePreset = Literal[
+    "ultrafast", "superfast", "veryfast", "faster", "fast", "medium", "slow", "slower", "veryslow"
+]
+
+
+def choices_of(annotation: object) -> tuple[str, ...]:
+    """The allowed values of a choice-typed setting, or () for any other type."""
+    return tuple(get_args(annotation)) if get_origin(annotation) is Literal else ()
+
+
 class Settings(BaseSettings):
     model_config = SettingsConfigDict(env_file=".env", extra="ignore")
 
@@ -85,7 +105,7 @@ class Settings(BaseSettings):
     search_daily_limit_per_user: int = 70
     # Source for live YouTube search: "scrape" (InnerTube, no API quota) or "api" (search.list,
     # 100 units/page). Scrape is the default; flip to "api" if scraping is blocked/breaks.
-    search_source: str = "scrape"
+    search_source: SearchSource = "scrape"
     # Recent-first backfill: how far back to fetch on the first pass per channel.
     backfill_recent_max_videos: int = 100
     backfill_recent_max_days: int = 365
@@ -221,7 +241,7 @@ class Settings(BaseSettings):
     audit_retention_days: int = 180
     audit_gc_minutes: int = 1440
     # On-disk layout: "plex" (Channel/Season YYYY/… [id].ext + .nfo/poster) or "flat".
-    download_layout: str = "plex"
+    download_layout: DownloadLayout = "plex"
     # Default SponsorBlock (skip sponsor segments) for new custom profiles.
     sponsorblock_default: bool = False
 
@@ -230,10 +250,10 @@ class Settings(BaseSettings):
     # startup and on change without a restart. This is what a process logs at before it can reach
     # the database — and the only knob the worker/runner children read, since they are short-lived.
     # TRACE (below DEBUG) is the "Teljes" diagnostics level; see app/logs.py.
-    log_level: str = "INFO"
+    log_level: LogLevel = "INFO"
     # "json" (one object per line — what Loki and the log_events sink ingest) or "text" (a readable
     # line for a local terminal). Redaction applies to both.
-    log_format: str = "json"
+    log_format: LogFormat = "json"
 
     # --- Plex integration (optional) ---
     # The Plex module plays the LOCAL physical media file directly (our own ffmpeg/direct-serve);
@@ -276,7 +296,7 @@ class Settings(BaseSettings):
     # Hardware-encoder policy: "auto" = VAAPI (h264_vaapi) when the iGPU render node is present AND the
     # source codec is hardware-decodable, else libx264 software; "on" = force VAAPI wherever the render
     # node exists (skip the decodability gate); "off" = always software. See plex/stream.py `_select_encode`.
-    plex_transcode_hw: str = "auto"
+    plex_transcode_hw: TranscodeHw = "auto"
     # Software-fallback libx264 tuning (IGNORED on the VAAPI path). `preset` trades CPU for size; `crf`
     # trades quality for size (lower = better + bigger). `max_height` caps the SOFTWARE transcode's output
     # height (0 = keep source) so a CPU re-encode of a tall source stays affordable — the GPU path always
@@ -286,7 +306,7 @@ class Settings(BaseSettings):
     # call, not a fix. Only affects files that go through remux/transcode — a direct-played file is
     # served untouched, but those are browser-native stereo anyway, not the 5.1 mixes this is for.
     plex_audio_loudnorm: bool = False
-    plex_transcode_preset: str = "veryfast"
+    plex_transcode_preset: TranscodePreset = "veryfast"
     plex_transcode_crf: int = 20
     plex_transcode_max_height: int = 1080
     # Where on-the-fly HLS remux segments are written (transient, reaped). Empty → a `.plex-hls`
@@ -335,6 +355,18 @@ class Settings(BaseSettings):
         https OAUTH_REDIRECT_URL as the signal for 'real deployment' vs local dev."""
         return self.oauth_redirect_url.lower().startswith("https://")
 
+    @field_validator("*", mode="before")
+    @classmethod
+    def _canonical_choice(cls, value: object, info: ValidationInfo) -> object:
+        """A choice setting accepts its value in any letter case (`LOG_LEVEL=debug`) and stores the
+        canonical spelling; an unknown value still fails the boot with pydantic's own message."""
+        field = cls.model_fields.get(info.field_name or "")
+        options = choices_of(field.annotation) if field else ()
+        if options and isinstance(value, str):
+            folded = value.strip().casefold()
+            return next((o for o in options if o.casefold() == folded), value)
+        return value
+
     @model_validator(mode="after")
     def _enforce_production_secrets(self) -> "Settings":
         """Fail fast rather than boot a public instance with the shipped placeholder