Add data dumps

[ArtOfCode-]

Add data dumps. Weekly export of the entire database minus anything sensitive, uploaded to S3 and made available via a new page linked in the footer. Also adds an option for manually-created dump records, intended for quarterly uploads to Archive.org.

Incorporates #1950 by cherry-pick.

[codecov]

Codecov Report

❌ Patch coverage is 86.44068% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.90%. Comparing base (9c10754) to head (f64ea30).

Files with missing lines	Patch %	Lines
app/jobs/data_dump_job.rb	88.88%	5 Missing ⚠️
app/controllers/dumps_controller.rb	60.00%	2 Missing ⚠️
app/models/dump.rb	87.50%	1 Missing ⚠️

Additional details and impacted files

Components	Coverage Δ
controllers	76.02% <60.00%> (-0.06%)	⬇️
helpers	85.32% <100.00%> (+0.01%)	⬆️
jobs	71.85% <88.88%> (+4.97%)	⬆️
models	93.02% <87.50%> (-0.03%)	⬇️
tasks	61.11% <ø> (ø)

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[cellio]

After switching to the branch and running the migrations, and before running any job to generate dumps, I hit the "data dumps" link in the footer and got:

ActionView::Template::Error (undefined method `title' for nil):

Causes:
NoMethodError (undefined method `title' for nil)
    29:   </thead>
    30:   <tbody>
    31:     <tr>
    32:       <td><%= @latest.title %></td>
    33:       <td><%= @latest.file.filename %></td>
    34:       <td><%= @latest.created_at.strftime('%Y-%m-%d') %></td>
    35:       <td><%= link_to 'Download', rails_blob_path(@latest.file, disposition: 'attachment') %></td>
  
app/views/dumps/index.html.erb:32

Let's defend against that; even if we intend to generate a dump immediately after deploying, (a) there might be a delay and (b) we can't assume other instances will do so.

[cellio]

Running the job from the Rails console produced an access error -- looks like we're missing a grant for perform.

:001> DataDumpJob.perform_later
Performing DataDumpJob (Job ID: 173732ce-812a-4dae-bd88-b6e324228c46) from Inline(default) enqueued at 2026-05-15T03:01:18.998364424Z
Error performing DataDumpJob (Job ID: 173732ce-812a-4dae-bd88-b6e324228c46) from Inline(default) in 17.49ms: ActiveRecord::ConnectionFailed (Mysql2::Error::ConnectionError: Access denied for user 'qpixel'@'%' to database 'qpixel_dump'):

(Stack trace follows; I assume you don't need all that.)

[ArtOfCode-]

Running the job from the Rails console produced an access error -- looks like we're missing a grant for perform.

I deliberately didn't include this as part of the job, as the DB user running the export shouldn't have GRANT permissions. I did it manually - I'll add some error handling for that specifically that puts a warning in the job log.