Add list_subjects MCP tool with subject aliases - #608

Summary

• New read-only tool list_subjects: the user's subjects from their own school data (one entry per class and school account) with the teacher where the school passes it on, every name the subject goes by, matching Teams channels and OneNote notebooks or sections (confidence exact, alias or fuzzy), and the next and last assessment. An optional subject returns only matching subjects, whatever name or language the user used, and echoes the search names (query.aliases) so they can be passed to search as terms.
• SubjectAliases is the shared name table: common Swiss subjects in DE/FR/IT/EN with abbreviations (Mathe / Mathematik / MA / maths, W&R / WuR, GP, physique / fisica, ABU, EPS and more) plus normalisation of case, accents, punctuation, &, and year or class prefixes (Mathe 4. LJ, BM23d Mathematik). Abbreviations of up to three letters only match a whole name.
• Permissions per source: the tool needs School accounts read; exams need Exams read, the grade fallback for the last assessment needs Grades read, teacher names (from lesson notes) need Calendar read, Teams channels need Drive read (same as list_teams), OneNote needs OneNote read. What is not allowed is left out and listed in skipped.
• Bounded: at most 12 teams get a channel lookup and 8 notebooks a section lookup, 4 calls in parallel, 20 second timeout; partial is set when a limit hits or a source fails.
• Helply's prompt now resolves the subject with list_subjects first and keeps the manual lookup as fallback.

Closes #603