Add historical comparison charts to stats page

[ArnavBallinCode]

Add Historical Comparison Charts to Stats Page

Implements #287

Added interactive bar charts to the stats page that compare PyLadiesCon's progress across the last 3 years (2023, 2024, and 2025), allowing organizers and the community to visualize the conference's growth over time.

Charts Included

The following 7 bar charts are displayed comparing 2023, 2024, and 2025:

1. Registrations Over the Years - Number of conference registrations
2. Proposals Over the Years - Number of talk/workshop proposals submitted
3. Number of Sponsors Over the Years - Count of sponsors
4. Sponsorship Amount Over the Years - Total sponsorship funds in USD
5. Number of Individual Donors Over the Years - Count of individual donors
6. Donation Amount Over the Years - Total donation funds in USD
7. Total Proceeds Over the Years - Combined sponsorship + donation amounts

• 2023 & 2024 Data: Hardcoded from official blog post (https://conference.pyladies.com/2024-pyladiescon-ends/)
• 2025 Data: Dynamically fetched from the database in real-time, including:
  • Registrations: PretixOrder model
  • Sponsors: SponsorshipProfile model
  • Sponsorship amounts: Calculated from sponsorship tiers and overrides
  • Donors: IndividualDonation model + Pretix donations
  • Donation amounts: Aggregated from both sources

Closes #287

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[Mariatta]

Thank you for the PR. It looks good so far, and I will do one final review later on. But this is very helpful.

[Mariatta]

Could this be added to /portal/static/styles.css?

[Mariatta]

Thank you. This looks great!