feat(n8n): add school paperless intake pipeline

docs/reference/SCHOOL_INTAKE_POSTGRES_SCHEMA.sql

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+-- Shared Postgres schema for Telegram-driven school-work intake.
+-- Apply in a dedicated `school_intake` database or adapt to an existing shared DB.
+
+CREATE TABLE IF NOT EXISTS school_paperless_intake (
+  intake_id              TEXT PRIMARY KEY,
+  class_name             TEXT NOT NULL,
+  assignment_name        TEXT NOT NULL,
+  submission_kind        TEXT NOT NULL,
+  semester               TEXT,
+  course_code            TEXT,
+  paper_kind             TEXT,
+  notes                  TEXT,
+  source                 TEXT NOT NULL DEFAULT 'telegram',
+  telegram_chat_id       TEXT,
+  telegram_message_id    TEXT,
+  original_filename      TEXT NOT NULL,
+  stored_filename        TEXT NOT NULL,
+  mime_type              TEXT NOT NULL,
+  file_size_bytes        BIGINT NOT NULL,
+  checksum_sha256        TEXT NOT NULL,
+  status                 TEXT NOT NULL DEFAULT 'received',
+  paperless_task_id      TEXT,
+  paperless_document_id  BIGINT,
+  paperless_title        TEXT,
+  received_at            TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  uploaded_at            TIMESTAMPTZ,
+  enriched_at            TIMESTAMPTZ,
+  error_text             TEXT,
+  created_at             TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  updated_at             TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_school_paperless_intake_status ON school_paperless_intake(status);
+CREATE INDEX IF NOT EXISTS idx_school_paperless_intake_doc_id ON school_paperless_intake(paperless_document_id);
+CREATE INDEX IF NOT EXISTS idx_school_paperless_intake_assignment ON school_paperless_intake(class_name, assignment_name);
+CREATE INDEX IF NOT EXISTS idx_school_paperless_intake_checksum ON school_paperless_intake(checksum_sha256);
+
+CREATE TABLE IF NOT EXISTS school_paperless_intake_events (
+  id              BIGSERIAL PRIMARY KEY,
+  intake_id       TEXT NOT NULL REFERENCES school_paperless_intake(intake_id) ON DELETE CASCADE,
+  event_type      TEXT NOT NULL,
+  event_payload   JSONB NOT NULL DEFAULT '{}'::jsonb,
+  created_at      TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_school_paperless_intake_events_intake_id
+  ON school_paperless_intake_events(intake_id);