fix: merge migrate_0.5.0.sql into migrate_0.4.9.9.sql

version_compare('0.5.0', '0.4.9.9', '<=') is FALSE in PHP,
so migrate_0.5.0.sql would never execute when upgrading to 0.4.9.9.
Merged its contents (sharing settings, plugin_hooks unique index)
into migrate_0.4.9.9.sql. All statements are idempotent.

Author: fabiodalez-dev

installer/database/migrations/migrate_0.4.9.9.sql

@@ -26,3 +26,52 @@ SET @sql = IF(@col_exists = 0,
 PREPARE stmt FROM @sql;
 EXECUTE stmt;
 DEALLOCATE PREPARE stmt;
+
+-- =============================================================================
+-- Add default sharing providers setting
+-- =============================================================================
+-- Enables Facebook, X, WhatsApp and Email share buttons on the book detail page.
+-- Admins can customise the selection in Settings > Sharing.
+
+INSERT IGNORE INTO system_settings (category, setting_key, setting_value, updated_at)
+VALUES ('sharing', 'enabled_providers', 'facebook,x,whatsapp,email', NOW());
+
+-- =============================================================================
+-- Add unique index on plugin_hooks for atomic upsert registration
+-- =============================================================================
+-- Prevents duplicate hook rows from concurrent registerHooks() calls.
+-- First, deduplicate existing rows (keep the one with the smallest id).
+
+DELETE ph1
+FROM plugin_hooks ph1
+JOIN plugin_hooks ph2
+  ON ph1.plugin_id       = ph2.plugin_id
+ AND ph1.hook_name       = ph2.hook_name
+ AND ph1.callback_method = ph2.callback_method
+ AND ph1.id > ph2.id;
+
+-- Drop old 4-column unique key if it exists (included callback_class which
+-- is NOT part of the runtime identity — the dispatcher deduplicates by
+-- plugin_id + hook_name + callback_method only)
+SET @old_idx = (SELECT COUNT(*) FROM INFORMATION_SCHEMA.STATISTICS
+                WHERE TABLE_SCHEMA = DATABASE()
+                  AND TABLE_NAME = 'plugin_hooks'
+                  AND INDEX_NAME = 'uk_plugin_hook_callback');
+SET @sql = IF(@old_idx > 0,
+    'ALTER TABLE plugin_hooks DROP INDEX uk_plugin_hook_callback',
+    'SELECT 1');
+PREPARE stmt FROM @sql;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+
+-- Re-create with correct 3-column key
+SET @new_idx = (SELECT COUNT(*) FROM INFORMATION_SCHEMA.STATISTICS
+                WHERE TABLE_SCHEMA = DATABASE()
+                  AND TABLE_NAME = 'plugin_hooks'
+                  AND INDEX_NAME = 'uk_plugin_hook_callback');
+SET @sql = IF(@new_idx = 0,
+    'ALTER TABLE plugin_hooks ADD UNIQUE KEY uk_plugin_hook_callback (plugin_id, hook_name, callback_method)',
+    'SELECT 1');
+PREPARE stmt FROM @sql;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;

installer/database/migrations/migrate_0.5.0.sql

@@ -426,8 +426,7 @@ migrate_0.4.6.sql    # LibraryThing missing fields (dewey_wording, barcode, entr
 migrate_0.4.7.sql    # LibraryThing comprehensive migration (25+ fields, indexes, constraints)
 migrate_0.4.8.1.sql  # Import logs tracking system (import_logs table + composite index)
 migrate_0.4.8.2.sql  # Illustratore field, lingua expansion, language normalization, anno_pubblicazione signed
-migrate_0.4.9.9.sql  # descrizione_plain column (HTML-free search)
-migrate_0.5.0.sql    # Social sharing settings + descrizione_plain safety net
+migrate_0.4.9.9.sql  # descrizione_plain column, social sharing settings, plugin_hooks unique index
 ```
 
 See `installer/database/migrations/README.md` for detailed migration documentation.