feat: add collaborative interview scheduling

Add candidate decisions, co-interviewer accounts, scheduled interview inbox and calendar views, candidate-safe meeting links, shared documents, and private live notes backed by RabbitMQ.

Extend the disposable integration environment to validate PostgreSQL migrations, seeded users, interview lifecycle behavior, and RabbitMQ fanout.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/migrations/00006_next_stage_interviews.sql

@@ -0,0 +1,60 @@
+-- +goose Up
+ALTER TABLE users
+    ADD COLUMN display_name VARCHAR(200) NOT NULL DEFAULT '';
+
+UPDATE users
+SET display_name = split_part(email, '@', 1)
+WHERE display_name = '';
+
+ALTER TABLE invitations
+    ADD COLUMN outcome VARCHAR(32) NOT NULL DEFAULT 'pending'
+        CHECK (outcome IN ('pending', 'passed', 'failed'));
+
+CREATE TABLE scheduled_interviews (
+    id BIGSERIAL PRIMARY KEY,
+    job_id BIGINT NOT NULL REFERENCES job_postings(id) ON DELETE CASCADE,
+    invitation_id BIGINT NOT NULL UNIQUE REFERENCES invitations(id) ON DELETE RESTRICT,
+    participant_id BIGINT NOT NULL REFERENCES participants(id) ON DELETE RESTRICT,
+    creator_id BIGINT NOT NULL REFERENCES users(id) ON DELETE RESTRICT,
+    starts_at TIMESTAMPTZ NOT NULL,
+    location VARCHAR(500) NOT NULL,
+    candidate_token VARCHAR(64) NOT NULL UNIQUE,
+    shared_document TEXT NOT NULL DEFAULT '',
+    status VARCHAR(32) NOT NULL DEFAULT 'scheduled'
+        CHECK (status IN ('scheduled', 'started', 'completed', 'cancelled')),
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX scheduled_interviews_job_id_idx ON scheduled_interviews(job_id);
+CREATE INDEX scheduled_interviews_participant_id_idx ON scheduled_interviews(participant_id);
+
+CREATE TABLE interview_attendees (
+    id BIGSERIAL PRIMARY KEY,
+    interview_id BIGINT NOT NULL REFERENCES scheduled_interviews(id) ON DELETE CASCADE,
+    user_id BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    status VARCHAR(32) NOT NULL DEFAULT 'invited'
+        CHECK (status IN ('invited', 'accepted', 'declined')),
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    UNIQUE (interview_id, user_id)
+);
+
+CREATE INDEX interview_attendees_user_id_idx ON interview_attendees(user_id);
+
+CREATE TABLE interview_notes (
+    id BIGSERIAL PRIMARY KEY,
+    interview_id BIGINT NOT NULL REFERENCES scheduled_interviews(id) ON DELETE CASCADE,
+    author_id BIGINT NOT NULL REFERENCES users(id) ON DELETE RESTRICT,
+    body TEXT NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX interview_notes_interview_id_idx ON interview_notes(interview_id);
+
+-- +goose Down
+DROP TABLE IF EXISTS interview_notes;
+DROP TABLE IF EXISTS interview_attendees;
+DROP TABLE IF EXISTS scheduled_interviews;
+ALTER TABLE invitations DROP COLUMN outcome;
+ALTER TABLE users DROP COLUMN display_name;