fix(mariadb): Stabilisiere CTE 'modulenames' und COUNT-Logik

[spacepc-de]

Problem
The DASH block generates a complex SQL query using a CTE (modulenames) that merges names and timestamps from multiple activity modules.
When rendering (especially when determining the total number of pages), this SQL fails on MariaDB — Moodle displays “Error reading from database” and a stack trace pointing to the COUNT query.

Root Cause

• The CTE references many module tables (assign, bigbluebuttonbn, board, kanban, subsection, …).
  If one or more of these modules are not installed, MariaDB aborts the query.
• The CTE name was output as {modulenames}. Moodle’s curly-brace notation is meant for real tables, not CTEs — this caused incorrect prefix substitution.
• The original COUNT logic mixed DISTINCT, GROUP BY, and ORDER BY, or referenced the CTE via a derived subquery — combinations that MariaDB handles poorly.

Fixes / Changes

• Made COUNT more robust:
  • Removed unnecessary DISTINCT, GROUP BY, and ORDER BY.
  • Use direct WITH … SELECT COUNT(…) … instead of wrapping the CTE in a subquery.
• Cleaned up CTE name:
  • {modulenames} → modulenames and updated all references accordingly.
• Defensive CTE sanitization:
  • Joins to non-existent module tables are replaced with harmless derived tables that return NULL columns, preventing COALESCE expressions from failing.
  • If an installed module lacks the name or timemodified fields, references are rewritten to NULL.
• Added optional SQL debug logs (DATA and COUNT) to expose the final executed SQL in debugging mode.

Result

• No more “Error reading from database” on MariaDB when certain modules are missing.
• COUNT queries are now stable and compatible across MariaDB, MySQL, and PostgreSQL.

Notes

• Tested on MariaDB (please specify version if available).
• No schema or data changes — only SQL/Query-Builder logic updates.

[stefanscholz]

Thank you @spacepc-de — we'll review it internally and get back to you after that.