save finalState for runs

[taylordowns2000]

This PR saves finalState for a run, as a precursor to #4485.

When the worker sends run:complete, we now persist the run's final state by setting final_dataclip_id on the runs table. Previously final_state was only used for webhook response broadcasting, then discarded. This is a prerequisite for #4485 (using the run's final output as input for the next cron execution).

The handler accepts two mutually exclusive fields from the worker:

• final_dataclip_id — reuse an existing step output dataclip (single-leaf workflows, no duplication)
• final_state — a new map to persist as a separate dataclip (multi-leaf workflows with merged output)

Neither field is required — backward compatible with the current worker.

Changes

• Migration: Adds nullable final_dataclip_id FK on runs → dataclips
• Run schema: New belongs_to :final_dataclip; complete/2 casts the new field
• CompleteRun handler: resolve_final_dataclip/2 either reuses an existing ID or inserts a new dataclip. Respects save_dataclips: false.
• Run channel: Injects project_id into the run:complete payload so new dataclips can be associated with the correct project

Performance considerations

• Single-leaf workflows (most common): Worker sends final_dataclip_id pointing to the last step's output dataclip. Zero additional storage, zero additional writes — just sets the FK.
• Multi-leaf workflows: One extra dataclip INSERT per run. The merged body doesn't exist elsewhere, so this is net-new data.
• Transaction scope: CompleteRun now wraps in Repo.transact. For the ID-reuse path this adds no extra queries. For the final_state path it's one INSERT + one UPDATE in a short transaction.

How to test

1. Run any workflow — final_dataclip_id will be null until the worker is updated (backward compatible)
2. After worker change: run a single-leaf workflow, verify final_dataclip_id equals the last step's output_dataclip_id (no duplicate)
3. After worker change: run a branching workflow, verify final_dataclip_id points to a new dataclip with the merged output
4. mix test test/integration/web_and_worker_test.exs

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[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.49%. Comparing base (5d4762c) to head (53fab9b).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/lightning/runs/handlers.ex	76.92%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4529      +/-   ##
==========================================
- Coverage   89.55%   89.49%   -0.06%     
==========================================
  Files         425      425              
  Lines       20307    20318      +11     
==========================================
- Hits        18185    18184       -1     
- Misses       2122     2134      +12     

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[taylordowns2000]

@stuartc , easier to review this all as a single PR. closing in favor of #4531