refactor: config-driven libraries; drop folder-add UI

The frontend AddSourceFolderDialog let users register source roots
from inside the app, but with the bootstrap auto-creating one for
the /photos mount on first boot, the dialog was redundant in the
common case and confusing in every other (users had to know which
container path corresponded to their host directory). Going
config-driven matches Plex/Photoprism/Immich and matches the
mental model "the docker mount IS the library".

Frontend
- Deleted components/dialogs/AddSourceFolderDialog.tsx entirely.
- LeftSidebar drops the "+ Add Source Folder" button + bottom-bar
  layout, the addFolderMutation, the dead Plus action button on
  the (no-longer-existing) folders/heaps tree headers, and the
  Plus icon import.
- api.ts: removed sourceFolders.add(), library.browse(), and the
  BrowseChild / BrowseResponse types. The remaining sourceFolders
  surface is read-only (list + manual scan).
- LeftSidebar bottom strip is now just the "Scan all folders"
  button when there's at least one source root.

Backend
- Dropped POST /folders (no consumers) along with FolderCreate /
  FolderResponse pydantic models. The folders router header now
  documents the config-driven approach.
- Dropped GET /library/browse (no consumers). Removed the unused
  os/HTTPException/SourceRoot imports it brought in.
- cleanup_data_integrity now also walks the source roots and logs
  a warning for any whose path is missing on disk. Doesn't auto-
  delete (a missing path could be a temporarily unmounted drive)
  but surfaces enough hint to fix it. Returns the count in the
  summary dict alongside merged-duplicates.

Docs
- README "How libraries are managed" section rewritten to spell
  out that mounts ARE source roots, edit .env + restart, no UI for
  managing source roots. New "Changing or adding libraries"
  section walks through the typical edit-restart loop including
  the optional volume-nuke for a clean slate.
- "Adding more libraries" subsection covers multi-mount via
  edited compose with a note that auto-registration is roadmap.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/routers/library.py

@@ -1,21 +1,15 @@
 """
-Library API router for stats, scanning, and directory browsing
+Library API router for stats and scanning
 """
-import os
-from fastapi import APIRouter, Depends, HTTPException
+from fastapi import APIRouter, Depends
 from sqlalchemy import select, func
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.database import get_db
-from app.models import Photo, SourceRoot
+from app.models import Photo
 
 router = APIRouter()
 
-
-# Always-allowed root for the directory browser. Whatever the user mounts
-# as PHOTO_DIRS in .env shows up here.
-DEFAULT_LIBRARY_ROOT = "/photos"
-
 @router.get("/stats")
 async def get_library_stats(db: AsyncSession = Depends(get_db)):
     """Get library statistics"""
@@ -44,89 +38,6 @@ async def get_library_stats(db: AsyncSession = Depends(get_db)):
         "total_size_gb": round(size / (1024**3), 2) if size else 0
     }
 
-@router.get("/browse")
-async def browse_directory(
-    path: str = DEFAULT_LIBRARY_ROOT,
-    db: AsyncSession = Depends(get_db),
-):
-    """List the immediate child directories of `path` so the frontend can
-    render a folder picker. The path is validated to live under one of the
-    allowed roots so this can't be used to enumerate the container
-    filesystem:
-
-      - The default library mount (/photos)
-      - Any active SourceRoot the user has already added (and its subtree)
-
-    Returns:
-      {
-        "path": str,             # canonical (normalized) path
-        "parent": str | null,    # parent path if still inside an allowed root
-        "is_existing_root": bool # whether `path` is itself a SourceRoot
-        "children": [
-          { "name", "path", "is_existing_root" }, ...
-        ]
-      }
-    """
-    # Build the allowed-roots set: default mount + every active SourceRoot.
-    sr_result = await db.execute(
-        select(SourceRoot).where(SourceRoot.is_active == True)  # noqa: E712
-    )
-    source_roots = sr_result.scalars().all()
-    sr_paths = [os.path.normpath(sr.path) for sr in source_roots]
-    allowed_roots = {os.path.normpath(DEFAULT_LIBRARY_ROOT), *sr_paths}
-
-    canonical = os.path.normpath(path)
-
-    # Path must live under (or be) one of the allowed roots — prevents
-    # browsing /etc, /data/db, etc.
-    def under_allowed(p: str) -> bool:
-        for root in allowed_roots:
-            if p == root or p.startswith(root + os.sep):
-                return True
-        return False
-
-    if not under_allowed(canonical):
-        raise HTTPException(
-            status_code=403,
-            detail=f"Path is outside the allowed photo roots",
-        )
-
-    if not os.path.isdir(canonical):
-        raise HTTPException(status_code=404, detail=f"Not a directory: {canonical}")
-
-    # Build the child list — only directories, hidden entries (dotfiles)
-    # excluded.
-    sr_path_set = set(sr_paths)
-    try:
-        entries = sorted(os.listdir(canonical))
-    except OSError as e:
-        raise HTTPException(status_code=500, detail=f"Cannot read directory: {e}")
-
-    children = []
-    for entry in entries:
-        if entry.startswith('.'):
-            continue
-        child_path = os.path.join(canonical, entry)
-        if not os.path.isdir(child_path):
-            continue
-        children.append({
-            "name": entry,
-            "path": child_path,
-            "is_existing_root": child_path in sr_path_set,
-        })
-
-    # Compute parent path if it's still inside an allowed root.
-    parent = os.path.normpath(os.path.dirname(canonical))
-    parent_in_scope = parent != canonical and under_allowed(parent)
-
-    return {
-        "path": canonical,
-        "parent": parent if parent_in_scope else None,
-        "is_existing_root": canonical in sr_path_set,
-        "children": children,
-    }
-
-
 @router.post("/scan")
 async def trigger_scan():
     """Trigger full library re-scan"""