Document the H5P findings

docs/API.md gains the endpoint and what it returns, why `play` is the bigger
and lesser of the two, the element-by-id route found while probing, and the
per-library shapes with the traps in them — the string "true", the inline cloze
markers, the correct-option-first convention, and the UI subtrees that drown an
exercise in button labels.

CLAUDE.md's "no quiz of its own" note said a contentId was the only handle onto
the content. That was the reason nobody looked further, so it now says where to
look instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/LOCAL.md

@@ -107,9 +107,9 @@ node dist/bin/cli.js sync
 ## Run the test suites
 
 ```bash
-npm test              # 224 offline tests
+npm test              # 239 offline tests
 npm run smoke         # end-to-end against the live instance, live-only mode
-DATABASE_URL=… npm run smoke    # end-to-end with the index (89 checks with a WebUntis key)
+DATABASE_URL=… npm run smoke    # end-to-end with the index (93 checks with a WebUntis key)
 ```
 
 Store tests need a database and skip without one: