LPR improvements (#17289)

* config options

* processing in maintainer

* detect and process dedicated lpr plates

* create camera type, add manual event and save snapshot

* use const

* ensure lpr events are always detections, typing fixes

* docs

* docs tweaks

* add preprocessing and penalization for low confidence chars

frigate/config/classification.py

@@ -127,6 +127,11 @@ class LicensePlateRecognitionConfig(FrigateBaseModel):
 
 class CameraLicensePlateRecognitionConfig(FrigateBaseModel):
     enabled: bool = Field(default=False, title="Enable license plate recognition.")
+    expire_time: int = Field(
+        default=3,
+        title="Expire plates not seen after number of seconds (for dedicated LPR cameras only).",
+        gt=0,
+    )
     min_area: int = Field(
         default=1000,
         title="Minimum area of license plate to begin running recognition.",