feat(public-gallery): add public templates gallery API (Tasks 2 & 3)

Implements a no-auth read-only API for the public templates gallery page:
- Schemas in schemas/public_templates.py (PublicFlowTemplate, PublicScriptTemplate, PublicGalleryResponse, PublicFlowDetail, PublicScriptDetail)
- Five endpoints under /api/v1/public/templates: listing, flow detail, script detail, categories with counts, full-text search
- Tree preview truncated to 3 levels max; script_body never exposed
- Rate limited at 30/minute; paginated with category/type/sort filters
- 25 passing integration tests covering feature flags, truncation, script body protection, search, categories, and 404 behavior

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/schemas/public_templates.py

@@ -0,0 +1,83 @@
+"""Schemas for the public templates gallery. No auth required."""
+
+from datetime import datetime
+from typing import Any
+from uuid import UUID
+
+from pydantic import BaseModel
+
+
+class PublicFlowTemplate(BaseModel):
+    id: UUID
+    name: str
+    description: str | None = None
+    category: str | None = None
+    tree_type: str
+    step_count: int
+    usage_count: int
+    success_rate: float | None = None
+    tags: list[str] = []
+    preview_structure: dict[str, Any] | None = None
+    created_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class PublicScriptTemplate(BaseModel):
+    id: UUID
+    name: str
+    description: str | None = None
+    category_name: str | None = None
+    category_icon: str | None = None
+    complexity: str | None = None
+    tags: list[str] = []
+    parameter_count: int = 0
+    requires_elevation: bool = False
+    requires_modules: list[str] = []
+    usage_count: int = 0
+    is_verified: bool = False
+    created_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class PublicGalleryResponse(BaseModel):
+    flow_templates: list[PublicFlowTemplate]
+    script_templates: list[PublicScriptTemplate]
+    total_flows: int
+    total_scripts: int
+    categories: list[str]
+    domains: list[str]
+
+
+class PublicFlowDetail(BaseModel):
+    id: UUID
+    name: str
+    description: str | None = None
+    category: str | None = None
+    tree_type: str
+    step_count: int
+    usage_count: int
+    success_rate: float | None = None
+    tags: list[str] = []
+    preview_structure: dict[str, Any] | None = None
+    created_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class PublicScriptDetail(BaseModel):
+    id: UUID
+    name: str
+    description: str | None = None
+    category_name: str | None = None
+    complexity: str | None = None
+    tags: list[str] = []
+    parameters: list[dict[str, Any]] = []
+    requires_elevation: bool = False
+    requires_modules: list[str] = []
+    usage_count: int = 0
+    is_verified: bool = False
+    created_at: datetime
+
+    model_config = {"from_attributes": True}