fix(civitai): tolerate null availability, expose early access fields

The /model-versions endpoint now serializes availability as null, which
failed str validation and turned every version lookup into a 404. Coerce
null to the default like the creator username validator already does.

Pass through earlyAccessEndsAt and earlyAccessConfig so clients can see
that a download requires a Buzz purchase before requesting the file.

modules/civitai/models_civitai.py

@@ -58,6 +58,8 @@ class CivitVersion(BaseModel):
     base_model: str = Field("Unknown", alias="baseModel")
     published_at: str | None = Field(None, alias="publishedAt")
     availability: str = "Unknown"
+    early_access_ends_at: str | None = Field(None, alias="earlyAccessEndsAt")
+    early_access_config: dict | None = Field(None, alias="earlyAccessConfig")
     description: str | None = None
     trained_words: list[str] = Field(default_factory=list, alias="trainedWords")
     stats: CivitStats = Field(default_factory=CivitStats)
@@ -66,6 +68,13 @@ class CivitVersion(BaseModel):
     nsfw_level: int = Field(0, alias="nsfwLevel")
     download_url: str = Field("", alias="downloadUrl")
 
+    @validator('availability', pre=True)
+    def coerce_null_availability(cls, v): # pylint: disable=no-self-argument
+        # /model-versions/{id} serializes availability as null, which failed
+        # str validation and turned every version lookup into a 404. Fall back
+        # to the default instead of rejecting the whole version.
+        return "Unknown" if v in (None, "") else v
+
 
 class CivitCreator(BaseModel):
     class Config: