enhancement: competitions dataset isn't wired through studymap.config.ts like places is

[yakew7]

Problem

studymap.config.ts exists specifically so a self-hoster can point StudyMap at their own dataset without touching any other file — that's the whole point of #30 ("data-source abstraction") and #29 ("config-driven region"), and SELF-HOSTING.md step 3 says exactly that: "places: swap the sample imports for your own data/places/*.json files".

That abstraction was never extended to competitions. src/lib/competitions.ts imports every category file directly and hardcodes it:

import artsDesign from "../../data/competitions/arts_design.json";
import business from "../../data/competitions/business.json";
import coding from "../../data/competitions/coding.json";
// ...12 more direct imports, one per category file

There is no competitions field in StudyMapConfig (studymap.config.ts) the way there is a places field. A self-hoster following SELF-HOSTING.md's pattern for places has no equivalent lever for competitions — forking the repo for a different competitions catalog means editing src/lib/competitions.ts directly, not the one config file the rest of the self-host story is built around.

Fix

Mirror the places pattern: add a competitions: Competition[] field to StudyMapConfig in studymap.config.ts, move the 15 category imports there, and have src/lib/competitions.ts read from the config the way src/lib/places.ts reads places from it.

Acceptance criteria

• studymap.config.ts has a competitions field, populated the same way places is
• src/lib/competitions.ts no longer imports data/competitions/*.json directly
• SELF-HOSTING.md updated to mention swapping competitions the same way it already covers places
• Existing competitions tests still pass