feat(db): wal mode and inferred response shapes in block_schema_cache

[gandarfh]

What

Foundation for typed ref resolution by external read-only consumers (the language server):

• WAL journal mode on the app database: readers are never blocked by the app's writes (the default rollback journal could block either side). Regression test asserts the mode after init.
• block_schema_cache (migration 015): on every successful run that carries an alias, the result-save path derives the structural skeleton of the response JSON (objects keep keys, arrays keep one sampled element, scalars become type names — no data stored) and upserts it keyed by (file_path, alias). Rows carry cache_schema_version; readers treat a different version as a cache miss and the cache rebuilds on the next run instead of migrating in place.

Shape derivation is pure and unit-tested; an integration test covers the upsert/read roundtrip including latest-wins overwrite and version filtering. Full crate suite: 1107 green.

Consumers

The language server reads shapes to resolve {{alias.path}} fields (hover types, field completion, typo squiggles); the desktop app gains a refresh notification to the server in a follow-up.