fix: exercises add from external and public workout presets

[apedley]

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Description

This PR fixes two a bug with public workout sets (that are full of public exercises) not showing for other users.
I also discovered and fixed a bug where users couldn't privately have the same exercise from the same provider. For example, if user 1 has Bench Press from free exercise db, trying to add it on another user would result in a server error.

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PR Type

• [ x] Issue (bug fix)
• New Feature
• Refactor
• Documentation

Checklist

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New features only:

• [MANDATORY for new feature] Alignment: I have raised a GitHub issue and it was reviewed/approved by maintainers or it was approved on Discord.

Frontend changes (SparkyFitnessFrontend/ or src/):

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[gemini-code-assist]

Code Review

This pull request scopes external exercise sources to individual users by updating the database schema and modifying repository and service methods to include a userId parameter in lookups. It also updates the image resolution logic to use the canonical source for unauthenticated requests and adjusts RLS policies for workout presets. Feedback was provided regarding the image filename matching logic, which currently uses endsWith and could lead to ambiguous matches; using path.basename is recommended for better precision.