feat: add reusable SQL snippets with side panel in editor

[aalvaropc]

Summary

• New sql_snippets table with title, SQL, description (migration included)
• New Snippets context with CRUD operations scoped to workspace
• Side panel in SQL editor now has tabs: History | Snippets
• Create snippets from current editor SQL via "New Snippet" button
• Insert snippets into editor with one click (appends to current SQL)
• Delete snippets with confirmation
• Snippets persist per workspace and are shared among team members

Test plan

• mix precommit — 638 tests, 0 failures
• Manual: switch to Snippets tab, create a snippet, insert it into editor, delete it