fix(docker): decouple service builds to enable parallel execution

[ChefControl]

Summary

• Restructured manager and feeder Dockerfiles to copy only Cargo.toml manifests from sibling workspace members instead of the full source tree, with stub source files for workspace resolution
• Added per-Dockerfile .dockerignore files (BuildKit feature) to exclude irrelevant directories from each service's build context
• Added a cargo fetch --locked layer that caches dependency downloads separately from source compilation, improving rebuild times

How it works

Each Rust Dockerfile now:

1. Copies only Cargo.toml files from all workspace members (not source code)
2. Creates stub main.rs/lib.rs for members it doesn't compile
3. Runs cargo fetch --locked to cache dependencies in a separate layer
4. Copies only the real source for shared/ and the target service
5. Builds only the target package

The Dockerfile.dockerignore files ensure Docker only sends relevant files as build context — the manager build excludes frontend/ and feeder/src/, and vice versa. This eliminates I/O contention between concurrent builds.

Test plan

• Build manager image: docker build -t manager:test -f manager/Dockerfile .
• Build feeder image: docker build -t feeder:test -f feeder/Dockerfile .
• Build frontend image: docker build -t frontend:test frontend
• Build all three in parallel to verify no contention
• Deploy to minikube and verify services start correctly

Closes #41

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