Proposals overhaul

[marcoacierno]

Epic to keep track of the wide work that involves renaming Submissions to Proposals and converting all the custom "talks", panels, keynotes etc, to use the same Proposal system.

Moving keynotes to use the Proposals system will mean that keynoters will be able to edit the proposal via pycon.it, we will be able to upload slides using our proposal resources system and get all the improvements and features that we build for proposals.

### High-level work
- [ ] Fully rename all usages of Submission to Proposal
- [ ] Add ability to have custom session lengths that aren't exposed in UI (we don't want to show Keynote 1h in the list of durations available)
- [ ] ??

[marcoacierno]

@claude work on part 1 of this proposal, renaming everything named "Submission" to "Proposal"

[claude]

Claude encountered an error —— View job

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Working on renaming Submission to Proposal

Progress

• Rename models in backend/submissions/models.py
• Rename querysets and other files in backend/submissions/
• Update backend/api/submissions/ (types, schema, mutations, permissions)
• Update all imports across backend (voting, schedule, reviews, conferences, etc.)
• Update INSTALLED_APPS in settings
• Update ForeignKey/related_name references in other apps
• Rename directories with git mv
• Create database migration
• Update frontend code
• Run linters/formatters
• Commit and push

Now renaming directories...

--- | Branch: claude/issue-3888-20251213-1812