Fix backend↔DB sync for sessions & maps

[GerardPaligot]

Summary

The agenda/maps sync already used an "upsert + diff-delete" pattern, but the delete half was incomplete. Combined with enforced foreign keys, that produced three symptoms: stale data not removed, sync silently doing nothing, and duplicated map shapes/pictograms.

Root cause: when the backend dropped a referenced parent (category/format/tag/speaker), clean() deleted it while a stale junction row still pointed at it → SQLITE_CONSTRAINT_FOREIGNKEY → the swallowing catch in fetchAndStoreAgenda rolled back the whole transaction. So "stale data not removed" and "sync does nothing" were the same bug.

This PR completes the cleanup and makes deletion FK-safe:

• Maps (M1): clear shapes/pictograms per event before re-inserting — they have autoincrement PKs and can't be upserted, so they accumulated every sync (and the Map REPLACE itself tripped an FK once children existed).
• Sessions: rebuild SessionCategory/SessionFormat/SessionTag and speaker links for the response's sessions, delete child/junction rows before parents in clean(), wire up the unused Tag delete, add real EventSession/Schedule diff-deletes, and drop the misdirected diffSessions/deleteSessions duplicates.
• Favorites (F1): TalkSession now uses an INSERT OR IGNORE … UPDATE upsert (like Category/Format) so is_favorite survives a sync instead of being reset to false by INSERT OR REPLACE.
• Logging (X1): fetchAndStoreAgenda now logs the event id on failure (catch kept by design).
• Tests: new in-memory SQLDelight harness (desktopTest, FK enforcement on) covering duplication, idempotent re-sync, category reassignment without FK crash, favorite preservation, and removal of stale rows.

Test Plan

• ./gradlew :shared:core:desktopTest — 20 tests, 0 failures
• ./gradlew ktlintCheck detekt — clean
• ./gradlew lintDebug — clean
• Manual: sync an event, remove a session/category on the backend, re-sync, confirm it disappears
• Manual: favorite a talk, re-sync, confirm it stays favorited

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