feat(tasks): 阶段A后端核心 —— 多模式任务字段 + 接单资格判定 + 各模式限额守卫 + 公开广场/详情can_accept

- Task 增扩展列：publisher_role/visibility/c_visible/park_public/assign_type/assign_park_id/assign_opc_id/park_id/park_released/bid_quota/win_quota/eligibility(JSON)/settle_mode(默认operator_escrow)
- TaskRepository._to_dict/create/update 同步新字段；迁移 0020_task_multi_mode
- TaskService.can_accept(task,user)：仅被指派/园区专属/指派园区未发单 + eligibility(证书/地域/性别/工时/领域/专长/案例) 判定，返回 ok/reasons
- 抢单(grab)与投标(bid)接入 can_accept 守卫；bid 加报名上限与重复报名校验
- /opc/tasks 公开广场改为 published+c_visible；/opc/tasks/{id} 详情同；登录后附带 can_accept/accept_reasons(经 optional_current_user)

alembic/versions/0020_task_multi_mode.py

@@ -0,0 +1,48 @@
+"""多端任务体系：任务扩展列(发布方/可见性/指派/园区/竞标配额/接单资格/托管)
+
+Revision ID: 0020_task_multi_mode
+Revises: 0019_content_review
+Create Date: 2026-08-27
+"""
+from __future__ import annotations
+
+import sqlalchemy as sa
+from alembic import op
+
+revision = "0020_task_multi_mode"
+down_revision = "0019_content_review"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    cols = [
+        ("publisher_role", sa.String(), "server_default", ""),
+        ("visibility", sa.String(), "server_default", "public"),
+        ("c_visible", sa.Boolean(), "server_default", sa.text("1")),
+        ("park_public", sa.Boolean(), "server_default", sa.text("0")),
+        ("assign_type", sa.String(), "server_default", ""),
+        ("assign_park_id", sa.String(), "nullable", True),
+        ("assign_opc_id", sa.String(), "nullable", True),
+        ("park_id", sa.String(), "nullable", True),
+        ("park_released", sa.Boolean(), "server_default", sa.text("0")),
+        ("bid_quota", sa.Integer(), "server_default", "0"),
+        ("win_quota", sa.Integer(), "server_default", "0"),
+        ("eligibility", sa.Text(), "server_default", "{}"),
+        ("settle_mode", sa.String(), "server_default", "operator_escrow"),
+    ]
+    for name, typ, kw, val in cols:
+        if kw == "nullable":
+            op.add_column("tasks", sa.Column(name, typ, nullable=val))
+        elif kw == "server_default":
+            op.add_column("tasks", sa.Column(name, typ, nullable=False, server_default=val))
+
+
+def downgrade() -> None:
+    for name, *_ in [
+        ("publisher_role",), ("visibility",), ("c_visible",), ("park_public",),
+        ("assign_type",), ("assign_park_id",), ("assign_opc_id",), ("park_id",),
+        ("park_released",), ("bid_quota",), ("win_quota",), ("eligibility",),
+        ("settle_mode",),
+    ]:
+        op.drop_column("tasks", name)