Issue #3607988: Add a codebase metrics documentation page

Adds a Codebase metrics documentation page to the mkdocs site: a measured snapshot of the codebase and its submodules, counted with cloc.

• Lines by language, a production vs test split, code/comment/blank composition, a per-submodule ranking, a Test suite section (classes and methods by type), and an Architecture surface section (services, plugins, hooks, routes, entities, config schema). Static inline SVG charts, no JavaScript.
• Adds a nav entry and adds cloc to the cspell word list. Verified with mkdocs build --strict.

Snapshot (1.x): 8,268 lines of code across 102 files; production 4,874 / test 3,394; 24 test classes / 165 methods; 2 submodules (webdav_key, webdav_lock_pdo).