Add configurable DBContext capabilities

[MikeChristensen]

Summary

• add independently configurable ingredient autocomplete, NLP parsing, and recipe modeler capabilities
• retain the current all-capabilities default for backward compatibility
• provide database-backed recipe aggregation when the modeler is disabled
• decouple modeler ingredient names from the autocomplete index
• fail clearly when an API requires a disabled capability
• document profiles, measurements, and the legacy NLP view audit

Performance

Using the sample PostgreSQL data, the parsing-only profile reduced median startup time by approximately 37%, managed memory by 46%, and working set by 31% compared with the previous all-feature initialization.

Validation

• dotnet build core.slnx: succeeded with 0 warnings and 0 errors
• unit tests: 39 passed
• end-to-end PostgreSQL parsing-only flow: recipe search, detail, database-backed aggregation, NLP parsing, shopping-list persistence, and cleanup passed

Closes #9