feat: make Drizzle ORM the default for Lakebase plugin

[pkosiec]

Summary

Makes Drizzle ORM the default experience when scaffolding apps with the Lakebase plugin.

SDK

• Add appkit.lakebase.drizzle(schema) helper using dynamic import() — zero cost if unused, full NodePgDatabase<TSchema> types when drizzle-orm is installed
• drizzle-orm added as optional peerDep (no impact on users who don't use it)

Template

• Rewrite todo CRUD example from raw SQL to Drizzle query builder
• Add server/db/schema.ts (pgSchema + table definition + inferred types)
• Add server/db/index.ts (ensureSchema + initDb via SDK helper)
• Add drizzle.config.ts for drizzle-kit migrations
• Add conditional drizzle-orm + drizzle-kit deps (only when lakebase feature is selected)
• Add db:push, db:generate, db:migrate npm scripts

Docs

• Add "Drizzle ORM (default)" section to Lakebase plugin docs with schema, queries, migrations, OBO examples
• Expand @databricks/lakebase README Drizzle example

Key design decisions

• SDK helper vs template helper: The drizzle() method lives in the SDK (not template) because template code is frozen at scaffold time — SDK code stays current with npm update
• Dynamic import: drizzle-orm is loaded lazily via await import() so there's zero runtime cost for users who don't call .drizzle()
• Type inference: TypeScript generates a lazy import("drizzle-orm/node-postgres").NodePgDatabase<TSchema> in .d.ts — resolved only when the method is called
• RoutingPool compatibility: Drizzle checks constructor.name.includes('Pool') for transactions — RoutingPool passes this, so OBO routing works transparently
• Conditional template deps: drizzle-orm and drizzle-kit use Go template conditionals ({{if .plugins.lakebase}}) in package.json — only installed when lakebase is selected

Test plan

• SDK builds and typechecks cleanly
• Template renders correctly via databricks apps init --template
• All rendered Drizzle files are clean (no Go template syntax)
• Existing tests pass (2527/2530, 3 pre-existing failures)
• E2E: scaffold app with lakebase, deploy, verify CRUD operations work