feat: implement customer add handler

[alanshaw]

Implements the /customer/add handler, which allows Hilt to inject customers from fil.one into Sprue.

Depends on:

• feat: customer add command libforge#40

[Copilot]

Pull request overview

Adds support for the /customer/add UCAN command in Sprue so the service can accept customer records (e.g., from Hilt/fil.one) and persist them via the configured customer store implementation.

Changes:

• Implement /customer/add handler with subject validation (must target the service DID) and customer-store persistence.
• Add unit tests covering success, wrong-subject rejection, and duplicate-customer behavior.
• Wire the new handler into the Fx ucan_handlers group and bump libforge to the required version.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
pkg/service/handlers/customer_add.go	New /customer/add route handler validating invocation subject and writing to customer.Store.
pkg/service/handlers/customer_add_test.go	Tests for handler success, invalid subject failure, and duplicate customer failure.
internal/fx/service/handlers/provider.go	Registers NewCustomerAddHandler in the ucan_handlers Fx group.
go.mod	Updates github.com/fil-forge/libforge dependency version.
go.sum	Adds checksums for the updated libforge version.

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[bajtos]

I'd like to resolve the discussion in fil-forge/libforge#40 and fil-forge/hilt#3 before reviewing this one.

[alanshaw]

@bajtos GTG now?

[bajtos]