refactor(api): add response schemas to the thin domains

[harshtandiya]

Stacked on #295 — read that first. Diff here is only the four thin domains.

Endpoints now return pydantic models and throw named errors instead of hand-built dicts and generic 417s.

def get_user_info() -> UserInfoResponse | GuestInfoResponse:
def get_enabled_languages() -> list[LanguageOption]:
def get_campaign_details(campaign: str) -> CampaignResponse:
def get_my_proposals() -> list[ProposalListItem]:

Three modelling calls, all made to keep the wire byte-identical:

• get_user_info needs two models. Guest gets 2 keys, logged-in user gets 10. One model with optionals would add "name": null, "roles": null, … to the guest payload.
• Settings flags typed int, not bool. They're Check fields travelling as 0/1.
• roles stays an untyped list. It holds child-table documents that serialize as full docs.

Errors carry their own copy (base class is in #295):

class AlreadyRegistered(Conflict):
    title = _lt("Already Registered")
    message = _lt("You have already registered your interest in this campaign.")

AlreadyRegistered.throw()

Two things I deliberately left alone:

• CRMNotAvailable is 400, not 5xx — frappe/app.py:415 writes an Error Log for anything ≥ 500, and a missing app would log on every click.
• Login-required and campaign-not-found keep frappe.AuthenticationError / frappe.DoesNotExistError. Right status, clear message, wrapping adds nothing.
• register_campaign_interest still doesn't check campaign.enabled (only get_campaign_details does). Extracting a shared get_campaign() made adding it tempting; kept behaviour-neutral. Probably a real gap — separate call.

Checks: 22 tests green (2 campaign cases skip, Buzz Campaign won't insert without the crm app). Payloads captured before/after via orjson_dumps(..., default=json_handler) are byte-identical, including datetime as 2026-07-08 00:30:45.683135 rather than RFC-3339. ruff / pre-commit / full bench run-tests --app buzz clean.

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[harshtandiya]

Rebased onto updated #295 — the BuzzAPIError base moved there, since that PR introduces api/exceptions.py. This one now only has the per-domain error classes.

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[github-actions]

Backport failed for main, because it was unable to cherry-pick the commit(s).

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin main
git worktree add -d .worktree/backport-296-to-main origin/main
cd .worktree/backport-296-to-main
git switch --create backport-296-to-main
git cherry-pick -x 11e7d6feeb7c1c25f8ba9b2fd54914ecd8e2ba6d