Reorganize repo: backend/, deploy/, docs/ layout + AGENTS.md

Restructure the monorepo into clear top-level directories:
- backend/: Go module root (cmd, internal, pkg, configs, migrations,
  docs/swagger, scripts, tests, go.mod, Makefile, .air.toml)
- deploy/: Docker (Dockerfile, docker-compose*), quickstart, fluentd
- docs/: project documentation + reports/ (moved from repo root)
- AGENTS.md: new AI coding-agent guide at repo root

Update all references to the new layout:
- Dockerfile: COPY backend/go.mod, COPY backend/ (context = repo root)
- docker-compose files: context ../.., dockerfile deploy/docker/Dockerfile,
  env_file ../../.env, volume mounts ../../backend:/app
- deploy/quickstart/compose.yaml: dockerfile deploy/docker/Dockerfile
- CI: working-directory: backend for go commands, file deploy/docker/Dockerfile,
  coverage path backend/coverage.out, health_check backend/scripts/
- backend/Makefile: docker target uses -f ../deploy/docker/Dockerfile ../
- README: architecture tree, quickstart, config paths updated

Move root stray scripts (rename_models.*, run_m11_tests.sh, verify_build.sh,
gorm_bool_main.go) to backend/scripts/legacy/. All moves via git mv to
preserve history. Build, vet, SQLite tests, and docker compose config verified.

backend/migrations/000015_add_integration_hook_tables.up.sql

@@ -0,0 +1,49 @@
+-- G16: Third-party integration tables (IntegrationHook + IntegrationApp)
+-- Reference: Chatwoot Integrations::Hook + Integrations::App
+
+-- IntegrationHook: per-account/per-inbox integration binding
+-- Stores provider-specific settings in JSONB, keyed by hook_type (slack/shopify/linear/notion/webhook)
+CREATE TABLE IF NOT EXISTS integration_hooks (
+    id SERIAL PRIMARY KEY,
+    account_id INTEGER NOT NULL,
+    inbox_id INTEGER,                              -- NULL = account-level hook
+    hook_type VARCHAR(50) NOT NULL,                -- webhook/slack/shopify/linear/notion
+    status VARCHAR(20) DEFAULT 'active',           -- active/inactive
+    url VARCHAR(1024) DEFAULT '',                  -- webhook callback URL
+    access_token VARCHAR(256),                     -- API token for the hook (unique)
+    settings JSONB DEFAULT '{}',                   -- provider-specific config (Slack channel_id, Shopify shop_domain, etc.)
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    deleted_at TIMESTAMP WITH TIME ZONE
+);
+
+CREATE INDEX idx_integration_hooks_account_id ON integration_hooks(account_id) WHERE deleted_at IS NULL;
+CREATE INDEX idx_integration_hooks_inbox_id ON integration_hooks(inbox_id) WHERE deleted_at IS NULL;
+CREATE INDEX idx_integration_hooks_hook_type ON integration_hooks(hook_type) WHERE deleted_at IS NULL;
+CREATE UNIQUE INDEX idx_integration_hooks_access_token ON integration_hooks(access_token) WHERE deleted_at IS NULL AND access_token IS NOT NULL;
+CREATE INDEX idx_integration_hooks_deleted_at ON integration_hooks(deleted_at);
+
+-- IntegrationApp: catalog of supported integrations
+-- Each app is keyed by hook_type (one app per provider type)
+CREATE TABLE IF NOT EXISTS integration_apps (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(255) NOT NULL,
+    description VARCHAR(512) DEFAULT '',
+    hook_type VARCHAR(50) NOT NULL,                -- webhook/slack/shopify/linear/notion
+    icon VARCHAR(512) DEFAULT '',                  -- icon URL or CSS class
+    action_url VARCHAR(1024) DEFAULT '',           -- frontend action URL for setup page
+    enabled BOOLEAN DEFAULT TRUE,                  -- whether the integration is available for installation
+    created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    updated_at TIMESTAMP WITH TIME ZONE DEFAULT NOW()
+);
+
+CREATE UNIQUE INDEX idx_integration_apps_hook_type ON integration_apps(hook_type);
+
+-- Seed default integration apps
+INSERT INTO integration_apps (name, description, hook_type, icon, action_url, enabled) VALUES
+    ('Slack', 'Connect Slack channels for real-time notifications', 'slack', '/assets/integrations/slack.svg', '/app/accounts/{account_id}/integrations/slack', TRUE),
+    ('Shopify', 'Sync Shopify orders and customer data', 'shopify', '/assets/integrations/shopify.svg', '/app/accounts/{account_id}/integrations/shopify', TRUE),
+    ('Linear', 'Create Linear issues from conversations', 'linear', '/assets/integrations/linear.svg', '/app/accounts/{account_id}/integrations/linear', TRUE),
+    ('Notion', 'Export conversation summaries to Notion pages', 'notion', '/assets/integrations/notion.svg', '/app/accounts/{account_id}/integrations/notion', TRUE),
+    ('Webhook', 'Custom webhook for event-driven integrations', 'webhook', '/assets/integrations/webhook.svg', '/app/accounts/{account_id}/integrations/webhook', TRUE)
+ON CONFLICT (hook_type) DO NOTHING;