feat(vector): replace inline page-image payloads with chunk_bbox (Deck #76)

Per-chunk PDF page renders (~150–700 KB base64 PNG each) were the dominant
disk consumer in production, repeatedly tripping `No space left on device:
WAL buffer size exceeds available disk space` on welcomed-malamute Qdrant.

Replace the inline highlighted_page_image / highlighted_page_number /
highlight_count fields with a small `chunk_bbox` field:
list[(x0, y0, x1, y1)] of normalized [0, 1] floats, ~32 bytes per chunk.
Astrolabe (the only known consumer) renders the highlight client-side as
a percentage-positioned overlay on top of the existing /api/v1/pdf-preview
render-on-demand path (cbcoutinho/astrolabe#76).

- pdf_highlighter: new compute_chunk_bboxes_batch() that reuses the
  existing _find_chunk_bbox text-search path, skipping all pixmap/PIL/PNG
  work.
- processor: store chunk_bbox + chunk_bbox_page in the Qdrant payload,
  drop highlighted_page_image + friends, drop the base64 import.
- visualization /api/v1/chunk-context and auth/viz_routes: read
  chunk_bbox instead of highlighted_page_image.
- vector/__init__: stop eagerly re-exporting `processor`/`scanner` —
  fixes a pre-existing circular import (search.algorithms ->
  vector.placeholder -> vector/__init__ -> processor -> scanner ->
  server.semantic -> search.bm25_hybrid -> search.algorithms partial).
  Test suite that was broken on master (test_bm25_hybrid.py et al.) now
  collects and passes.
- scripts/purge_page_images.py: ad-hoc, idempotent migration that
  delete_payload's the legacy keys from existing points. No reindex
  required; legacy chunks render the page with no overlay.

Pairs with cbcoutinho/astrolabe#76. Frontend handles missing chunk_bbox
gracefully, so this can land in either order.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/app.py

@@ -125,12 +125,13 @@ from nextcloud_mcp_server.server import (
 )
 from nextcloud_mcp_server.server.auth_tools import register_auth_tools
 from nextcloud_mcp_server.server.oauth_tools import register_oauth_tools
-from nextcloud_mcp_server.vector import processor_task, scanner_task
 from nextcloud_mcp_server.vector.oauth_sync import (
     oauth_processor_task,
     user_manager_task,
 )
+from nextcloud_mcp_server.vector.processor import processor_task
 from nextcloud_mcp_server.vector.qdrant_client import get_qdrant_client
+from nextcloud_mcp_server.vector.scanner import scanner_task
 from nextcloud_mcp_server.vector.webhook_receiver import handle_nextcloud_webhook
 
 logger = logging.getLogger(__name__)