feat(outlook): support explicit shared mailbox users

[kai-osthoff]

Summary

• add optional Outlook config.user support for shared or delegated mailbox UPNs and Entra object IDs
• preserve /me paths and legacy calendar hashes when the option is absent
• centralize the Microsoft Graph calendar path for list, create, update, and delete
• configure Outlook before OAuth setup so explicit mailboxes request Calendars.ReadWrite.Shared in the handler-owned authorization URL
• document delegated Graph permissions, Exchange mailbox delegation, and reauthentication requirements

Validation

• go test ./internal/adapter/outlook_http ./internal/adapter -race -count=1
• make test
• go vet $(go list ./... | grep -v /vendor/)
• go run honnef.co/go/tools/cmd/staticcheck@2025.1.1 ./...
• go run github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.5.0 run
• make build
• HTTP transport tests cover /me and escaped /users/{user} paths across list/create/update/delete
• lifecycle tests parse the actual authorization URL and assert the shared scope is conditional

Real Graph check

Redacted local dry-runs exercised both normal-user and explicit Shared Mailbox configurations using temporary config/auth copies; production bridge files were not changed. The pre-feature cached token can reach delegated calendar data but Graph returns ErrorAccessDenied for Shared Mailbox writes and extension expansion. A controlled test event was removed after every attempt, leaving no residue.

The PR is draft until the delegated account completes interactive authorization for the newly requested Calendars.ReadWrite.Shared scope and the controlled create/update/delete checks in both directions finish at 0/0/0.

Closes #313