Rework addon system with composable core module and AST-based addon application

[renardeinside]

Summary

Implements #77 — a comprehensive rework of the addon/template system:

• Modular core/ package: Split monolithic core.py into core/ with config.py, factory.py, dependencies.py, headers.py, resources.py, static.py — addons no longer need to replace the entire file
• Python AST editing in Rust (py_edit): New module using ruff_python_parser for surgical code modifications — add_import(), add_class_member(), add_call_keyword() with 16 unit tests
• Composable backend addons: BackendAddonSpec + PythonEdit enum for declarative addon definitions that apply via AST edits instead of file overlay
• Rename stateful → lakebase: Restructured as composable addon with lakebase_lifespan and Dependencies.Session
• New mock addons: sql, serving-endpoint, genie — stub addons with dependency injection
• Backend-only template: Minimal template skips frontend entirely — no Vite, no bun, no [tool.apx.ui]
• Backend-only dev server: Optional frontend port, conditional UI router/health check, skip OpenAPI/bun in preflight
• New structured types: DatabricksAppsHeaders model, DatabricksResourcesConfig with env prefix
• Composable lifespans: create_app(lifespans=[...]) with _chain_lifespans() for stacking multiple backend addons

Test plan

• cargo check passes
• All 206 tests pass (0 failures)
• apx init with Minimal template creates backend-only project
• apx init with Essential template creates full-stack project
• apx dev start on backend-only project starts only backend
• apx dev apply lakebase applies addon via AST edits

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