[ghidalgo3]

Follow-up to #89.

Summary

Relocates the recipe recommendation algorithm from a single PostgreSQL function into the C# application, and adjusts the algorithm: the ownership signal is gone and a recipe must share at least 2 ingredients with the source to be recommended.

Why: the database is great at indexes and set-membership lookups, but the recommendation policy (scoring, weighting, thresholds, ranking) is easier to read, unit-test, and evolve in application code without a migration. The DB now exposes primitives; the app composes them.

Changes

Database — new migration 018_recommendation_primitives.sql:

• Drops cooktime.recommend_recipes.
• Adds four data-only primitives: get_candidate_ingredient_sets, get_user_favorite_recipe_ids, get_user_last_cooked, get_recipe_summaries.

Application — new RecommendationService:

• Owns the algorithm. Pure static ScoreCandidates(...) does Jaccard similarity, the >=2-overlap gate, weighting, and ranking with no DB access (directly unit-testable).
• CookTimeDB drops GetRecipeRecommendationsAsync, gains the four primitive fetch methods.
• Endpoint resolves RecommendationService; the service performs the existence check (null -> 404).

Algorithm changes

• Ownership signal removed entirely.
• A recipe must share >=2 ingredients with the source to appear at all; favorite/novelty only re-rank within that set.
• Weights renormalized to sum to 1.0: ingredient similarity 0.70, favorite 0.18, novelty 0.12.

Contract + frontend — ownedByUser / "Your recipe" removed from the DTO, the TS type, and the admin score breakdown.

Docs — blog post updated to describe the app-side algorithm, the overlap gate, and new weights.

Tests

• scripts/test -> 47/47 pass (43 existing + 4 new pure unit tests in TestRecommendationService).
• Rewrote the two recommendation integration tests for the new weights and gate.
• scripts/test --lint -> 0 errors.

Test plan

• On a recipe with neighbors sharing >=2 ingredients, recommendations appear; recipes with only 1 shared ingredient do not.
• Signed in as Administrator, the score breakdown rows sum to the displayed score and there is no "Your recipe" row.
• Anonymous users still get similarity-only recommendations.

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