refactor(imports): standardize backend subpath imports

[AlCalzone]

Standardizes every backend-module import on the #api/* subpath convention.

• Replaces deep relative and retired backend aliases with one source, test, and compiled-runtime mechanism.
• Removes redundant resolver configuration and prevents alternative backend import forms from returning.
• Review focus: compiled/bundled resolution and singleton module identity.

Part of #4722. Stacked on #4741.

[coveralls]

Coverage Report for CI Build 29584425278

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Coverage: 62.624%

Details

• Patch coverage: No coverable lines changed in this PR.

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Coverage Stats

Relevant Lines:	8568
Covered Lines:	5610
Line Coverage:	65.48%
Relevant Branches:	5203
Covered Branches:	3014
Branch Coverage:	57.93%
Branches in Coverage %:	Yes
Coverage Strength:	36.7 hits per line

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