feat: flag deprecated models and redesign admin models table

[MaxEriksson2000]

Summary

This PR combines three related improvements to completion model management:

1. Deprecated models are now clearly flagged in the admin UI.
2. The admin models experience has been redesigned to make lifecycle state and actions easier to understand.
3. The model migration lifecycle has been hardened so historical questions keep their original model attribution, while migrated or deleted models can still be cleaned up safely over time.

Why

We had two related problems in practice:

• Deprecated models were hard to spot and still looked too similar to normal models in admin views.
• Completion model migration treated historical question records like active configuration, which caused incorrect model attribution after migration and risked breaking usage analytics and audit trails.

This PR separates those concerns properly:

• active configuration can be migrated,
• historical question records stay attached to the original model,
• old models can be retired and eventually cleaned up once no references remain.

What Changed

1. Deprecated model status in admin

• Added explicit deprecated status indicators in the models table.
• Added stronger visual treatment for deprecated rows and cells.
• Added status icons/labels so deprecated and migrated models are easier to distinguish.
• Added warning and migration guidance in the model detail dialog.

2. Admin model management UX redesign

• Reworked the admin models table layout and interaction model.
• Added a richer model detail dialog with lifecycle-aware actions.
• Improved status visibility for enabled, deprecated, and migrated models.
• Updated model labels and toggles so migrated models are visibly non-active.

3. Safer model migration lifecycle

Questions are historical records and are never migrated. Active configuration is migrated to the target model, while questions keep their original completion_model_id.

graph LR
    subgraph migration["Migration: A → B"]
        A["Model A<br/>(source)"] -->|"active config migrated"| B["Model B<br/>(target)"]
        A -.->|"migrated_to_model_id"| B
    end

    subgraph preserved["Preserved references"]
        Q1["Question 1"] -.->|"completion_model_id = A"| A
        Q2["Question 2"] -.->|"completion_model_id = A"| A
        Q3["Question 3"] -.->|"completion_model_id = A"| A
    end

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• Questions are no longer treated as migratable entities.
• Historical questions keep their original completion_model_id.
• After a successful migration, the source model is marked with migrated_to_model_id instead of being deleted during the migration flow.
• Migrated models are excluded from availability checks so new assistants/apps/services cannot be created against them.
• Soft-delete is used for completion model deletion, with lifecycle cleanup handling eventual hard-delete when references are gone.

Safety layers:

Layer	Mechanism
Application	can_access blocks migrated + deleted models in all 3 paths
Service	is_completion_model_available() enforced in assistant/app/service creation
Space payload	Filters out migrated + deleted models
Frontend	Excludes migrated from target lists, space settings, locks enable switch
Database	RESTRICT FK on questions.completion_model_id and migrated_to_model_id

4. Lifecycle cleanup for retired models

Added a weekly cleanup worker (ARQ cron, Sunday 4 AM) that hard-deletes models once all references are gone.

graph LR
    Start(( )) --> Active
    Active -->|LiteLLM flags| Deprecated
    Active -->|admin deletes| SoftDeleted

    Deprecated -->|admin migrates| Migrated
    Migrated -->|"cleanup job"| CleanedUp

    SoftDeleted -->|"cleanup job"| CleanedUp

    Migrated -. "questions still exist" .-> Migrated
    SoftDeleted -. "questions still exist" .-> SoftDeleted

    CleanedUp["Cleaned Up<br/>(hard-deleted, history preserved)"]

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Cleanup eligibility — ALL must be true:

1. Marked by lifecycle state (deleted_at IS NOT NULL or migrated_to_model_id IS NOT NULL)
2. Zero questions referencing it
3. Zero active entity references (assistants, apps, services, spaces, non-deleted templates)
4. No other model has migrated_to_model_id pointing to it

Soft-deleted templates are reconciled (FK cleared) before hard-delete. IntegrityError from RESTRICT FK is classified as db_restrict (skip, not error).

5. Migration history and auditability

History records survive model cleanup via snapshot fields, not dead UUIDs.

graph TD
    subgraph before["Before cleanup"]
        H["Migration History<br/>from_model_id = A (FK)<br/>to_model_id = B (FK)<br/>from_model_name = gpt-3.5-turbo<br/>from_provider_type = openai"]
        MA["Model A"]
        MB["Model B"]
        H -->|FK| MA
        H -->|FK| MB
    end

    subgraph after["After cleanup removes Model A"]
        H2["Migration History<br/>from_model_id = NULL (SET NULL)<br/>to_model_id = B (FK)<br/>from_model_name = gpt-3.5-turbo ✓<br/>from_provider_type = openai ✓"]
        MB2["Model B"]
        H2 -->|FK| MB2
    end

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• from_model_name / to_model_name — snapshot of CompletionModels.name at migration time
• from_provider_type / to_provider_type — snapshot of provider for audit readability
• from_model_id / to_model_id — live FK, becomes NULL after cleanup (by design)
• Per-model history lookup works for live models; use tenant history or migration_id after cleanup

Semantics to Note

• Questions are historical events and are intentionally never migrated.
• from_model_id / to_model_id in migration history only represent live model rows.
• After hard-delete, model-specific history by old model id is no longer expected to work; history is instead preserved via the migration record and snapshot fields.
• This is intentional and should be considered part of the lifecycle model, not a regression.

Testing

Added and updated focused unit and integration coverage for:

• deprecated model enrichment and visibility
• migration validation
• question attribution invariants
• migrated/deleted model access control
• cleanup lifecycle behavior
• migration history semantics
• sysadmin completion model delete behavior

All 151 completion model + AI model tests pass.

Closes

Closes #316
Closes #317
Closes #320
Closes #323