feat(model-manager): add CivitAI browse view + fix civitai.red / 401

- New "Browse CivitAI" view: thumbnail grid with search, type/sort/period
  filters and NSFW toggle; click a model card to download it (per-card
  version picker for multi-version models). Cursor + page based "Load more".
- Backend: /api/civitai/search and /api/civitai/download endpoints; new
  civitai_search() catalog helper.
- Fix 401 on paste: recognize the civitai.red mirror (and any civitai.*
  host), normalize API calls to civitai.com, and always resolve the
  model-version so type + filename are auto-detected for every CivitAI URL.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

model-manager/app/main.py

@@ -169,6 +169,39 @@ async def start_download(body: DownloadIn) -> dict:
     return row or {"id": download_id}
 
 
+# ---- CivitAI catalog browse ----------------------------------------------
+
+@app.get("/api/civitai/search")
+async def civitai_search(
+    query: Optional[str] = None,
+    types: Optional[str] = None,   # comma-separated CivitAI types
+    sort: Optional[str] = "Most Downloaded",
+    period: Optional[str] = "AllTime",
+    nsfw: bool = False,
+    page: Optional[int] = None,
+    cursor: Optional[str] = None,
+) -> dict:
+    if not db.get_setting("civitai_api_key"):
+        raise HTTPException(400, "Set your CivitAI API key in Settings first.")
+    type_list = [t for t in (types or "").split(",") if t] or None
+    try:
+        return await registries.civitai_search(
+            query=query, types=type_list, sort=sort, period=period,
+            nsfw=nsfw, page=page, cursor=cursor)
+    except Exception as exc:  # noqa: BLE001 - surface API errors to the UI
+        raise HTTPException(502, f"CivitAI search failed: {exc}")
+
+
+class CivitaiDownloadIn(BaseModel):
+    version_id: int
+
+
+@app.post("/api/civitai/download")
+async def civitai_download(body: CivitaiDownloadIn) -> dict:
+    url = f"https://civitai.com/api/download/models/{body.version_id}"
+    return await start_download(DownloadIn(url=url))
+
+
 @app.delete("/api/downloads/{download_id}")
 def delete_download(download_id: int) -> dict:
     row = db.get_download(download_id)