feat: ALTER TABLE DDL generation for all database plugins

[datlechin]

Summary

• Implement ALTER TABLE DDL generation (add/modify/drop columns, indexes, foreign keys, primary keys) across 8 database plugins: MySQL, PostgreSQL, SQLite, ClickHouse, MSSQL, DuckDB, Cassandra, Oracle
• Add granular schema editing capability flags (supportsAddColumn, supportsModifyColumn, supportsDropColumn, supportsRenameColumn, supportsAddIndex, supportsDropIndex, supportsModifyPrimaryKey) to DriverPlugin protocol
• Gate add/delete operations in Structure tab UI per database type capabilities
• Fix pre-existing supportsDropDatabase sync gap between Plugins and Packages copies of DriverPlugin.swift

Closes #803

Details

MySQL: CHANGE COLUMN (rename) / MODIFY COLUMN (same name), full column definition rebuild reusing buildColumnDefinitionSQL

PostgreSQL: Multi-statement modify — diffs old vs new column, emits only changed properties (RENAME COLUMN, ALTER COLUMN TYPE, SET/DROP NOT NULL, SET/DROP DEFAULT, COMMENT ON COLUMN)

SQLite: Limited to natively supported operations — ADD COLUMN, RENAME COLUMN, DROP COLUMN, CREATE/DROP INDEX. Modify column type/nullable/default returns nil (no table rebuild).

ClickHouse: ADD/MODIFY/DROP COLUMN with Nullable() wrapping, data-skipping index support. No FK/PK (immutable ORDER BY).

MSSQL: Complex modify — rename via sp_rename first, dynamic SQL to discover and drop default constraints, then ALTER COLUMN, then add new default constraint.

DuckDB: PostgreSQL-compatible syntax, full support.

Cassandra/ScyllaDB: ADD/DROP column only (CQL limitations).

Oracle: Full support with Oracle-specific syntax (ADD (col TYPE), MODIFY (col TYPE), RENAME COLUMN).

Test plan

• MySQL: add column, modify column (type + rename), drop column, add/drop index, add/drop FK, modify PK
• PostgreSQL: same matrix, verify schema-qualified names, verify multi-statement modify generates correct SQL in preview
• SQLite: add column, rename column, drop column, add/drop index. Verify modify type/nullable is blocked
• ClickHouse: add/modify/drop column, add/drop index. Verify FK/PK operations are blocked
• MSSQL: full matrix, verify default constraint handling on modify
• Verify SQL preview popover shows correct DDL before execution
• Verify unsupported operations are disabled in context menus (e.g., SQLite "Add Foreign Key")