feat: add project file system with CRUD, upload, inline editing, and video attachment support

- Add new `/projects/{slug}/files` endpoint group for per-project filesystem operations including directory and file CRUD, upload, and inline editing with public read and owner write access
- Extend attachment system to support video formats (webm, ogv, mov, m4v) with proper file icons and MIME types
- Implement configurable allowed file types via `allowed_file_types` site setting, replacing hardcoded `ALLOWED_UPLOAD_TYPES` with dynamic `allowed_extensions()` and `is_extension_allowed()` functions
- Add `delete_all_project_files()` call in `delete_content_item()` to clean up project files when a project is deleted
- Create database indexes on `project_files` table for `(project_uid, path)` and `(project_uid, parent_path)` to optimize file lookups
- Introduce `docs_prose.py` module with `render_prose()` function that renders Markdown content inside `data-render` divs using mistune, enabling dynamic prose rendering in documentation pages
- Enhance docs search with Markdown-aware text stripping (`_demarkdown()`) and improved HTML/script/style sanitization for better search indexing
- Update documentation API samples to reflect new attachment response fields (`is_image`, `is_video`, `mime_type`) and note video format support
- Update README to document the new project files endpoint and clarify AI gateway attribution for guest Devii sessions

tests/conftest.py

@@ -265,6 +265,18 @@ def bob(browser, seeded_db):
     ctx.close()
 
 
+@pytest.fixture
+def mobile_page(browser, seeded_db):
+    ctx = browser.new_context(viewport={"width": 412, "height": 840}, has_touch=True, permissions=["clipboard-read", "clipboard-write"])
+    p = ctx.new_page()
+    p.set_default_timeout(15000)
+    p.bring_to_front()
+    login_user(p, seeded_db["alice"])
+    yield p, seeded_db["alice"]
+    p.close()
+    ctx.close()
+
+
 @pytest.fixture(scope="session")
 def local_db():
     from devplacepy.database import init_db, db