feat(monitoring): add Redis exporter + Sentry docs to deployment guide

- Add redis-exporter container to docker-compose (oliver006/redis_exporter, 32MB)
- Add Redis scrape target to Prometheus config
- Add 4 Redis alert rules: RedisDown, HighMemory, HighConnections, RejectedConnections
- Document Step 19b (Sentry Error Tracking) in Hetzner deployment guide
- Document Step 19c (Redis Monitoring) in Hetzner deployment guide
- Update resource budget and port reference tables

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/development/migration/database-migrations.md

@@ -213,13 +213,13 @@ For complex changes that require data transformation:
    def upgrade() -> None:
        # Create new column
        op.add_column('products', sa.Column('normalized_price', sa.Numeric(10, 2)))
-       
+
        # Migrate data
        connection = op.get_bind()
        connection.execute(
            text("UPDATE products SET normalized_price = CAST(price AS NUMERIC) WHERE price ~ '^[0-9.]+$'")
        )
-       
+
        # Make column non-nullable after data migration
        op.alter_column('products', 'normalized_price', nullable=False)
 
@@ -329,7 +329,7 @@ def upgrade() -> None:
     if context.get_x_argument(as_dictionary=True).get('dev_data', False):
         # Add development sample data
         pass
-    
+
     # Always apply schema changes
     op.create_table(...)
 ```
@@ -345,20 +345,20 @@ For large data transformations, use batch processing:
 ```python
 def upgrade() -> None:
     connection = op.get_bind()
-    
+
     # Process in batches to avoid memory issues
     batch_size = 1000
     offset = 0
-    
+
     while True:
         result = connection.execute(
             text(f"SELECT id, old_field FROM products LIMIT {batch_size} OFFSET {offset}")
         )
         rows = result.fetchall()
-        
+
         if not rows:
             break
-            
+
         for row in rows:
             # Transform data
             new_value = transform_function(row.old_field)
@@ -366,7 +366,7 @@ def upgrade() -> None:
                 text("UPDATE products SET new_field = :new_val WHERE id = :id"),
                 {"new_val": new_value, "id": row.id}
             )
-        
+
         offset += batch_size
 ```