feat: use local blob storage

docs/TASKS/platform-scaffold/003-use-local-blob-storage.md

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+# Task: Use local blob storage
+
+## Feature
+
+`docs/FEATURES/platform-scaffold.md`
+
+## Goal
+
+Store uploaded portraits and logos on the API server filesystem instead of Azure Blob Storage.
+
+## Context
+
+User, client, and workspace portrait uploads already flow through `IBlobStorage`. The implementation can change without altering endpoint contracts or frontend behavior.
+
+## Files Likely To Change
+
+- `backend/src/Socialize.Api/Infrastructure/DependencyInjection.cs`
+- `backend/src/Socialize.Api/Infrastructure/BlobStorage/Services/*`
+- `backend/src/Socialize.Api/Infrastructure/BlobStorage/Configuration/*`
+- `backend/src/Socialize.Api/Program.cs`
+- `backend/src/Socialize.Api/appsettings.Development.json`
+
+## Constraints
+
+- Do not change API request or response contracts.
+- Keep upload validation behavior consistent with the existing blob storage implementation.
+- Serve returned blob URLs from the API host so the existing frontend can keep using `portraitUrl` and `logoUrl`.
+
+## Done When
+
+- [x] `IBlobStorage` resolves to local filesystem storage by default.
+- [x] Uploaded files are served back from the API host.
+- [x] Backend build passes.
+
+## Validation Commands
+
+```bash
+dotnet build backend/Socialize.slnx
+dotnet test backend/Socialize.slnx
+```