refactor: contain backend feature mappings

docs/TASKS/platform-scaffold/002-contain-backend-feature-mapping.md

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+# Task: Contain backend feature mapping
+
+## Feature
+
+`docs/FEATURES/platform-scaffold.md`
+
+## Goal
+
+Move backend feature-specific persistence mapping out of the shared `AppDbContext` body and into the owning `Modules/<Feature>/Data` folders.
+
+## Context
+
+Architecture docs state that current backend feature code stays under `Modules/<Feature>`. `AppDbContext` remains the shared EF Core composition point, but feature-owned model configuration should live with the feature entities.
+
+## Files Likely To Change
+
+- `backend/src/Socialize.Api/Data/AppDbContext.cs`
+- `backend/src/Socialize.Api/Modules/*/Data/*`
+
+## Constraints
+
+- Do not change API contracts.
+- Do not change table names, indexes, or column constraints.
+- Do not introduce a broader persistence refactor.
+
+## Done When
+
+- [x] Feature entity mappings live under the owning module folders.
+- [x] `AppDbContext` delegates feature configuration to modules.
+- [x] Backend build passes.
+
+## Validation Commands
+
+```bash
+dotnet build backend/Socialize.slnx
+dotnet test backend/Socialize.slnx
+```