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/000034_align_calls_whatsapp_fields.up.sql

@@ -0,0 +1,35 @@
+CREATE TABLE IF NOT EXISTS calls (
+    id SERIAL PRIMARY KEY,
+    account_id INTEGER NOT NULL,
+    inbox_id INTEGER,
+    conversation_id INTEGER NOT NULL,
+    contact_id INTEGER,
+    message_id INTEGER,
+    accepted_by_agent_id INTEGER,
+    provider VARCHAR(50) DEFAULT 'twilio',
+    direction VARCHAR(50),
+    provider_call_id VARCHAR(255),
+    conference_sid VARCHAR(255),
+    caller_type VARCHAR(100) NOT NULL DEFAULT '',
+    caller_id INTEGER NOT NULL DEFAULT 0,
+    status VARCHAR(50) NOT NULL DEFAULT 'ringing',
+    duration INTEGER DEFAULT 0,
+    call_direction VARCHAR(50) NOT NULL DEFAULT '',
+    recording_url VARCHAR(512),
+    additional_attributes JSONB DEFAULT '{}',
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    updated_at TIMESTAMPTZ DEFAULT NOW(),
+    deleted_at TIMESTAMPTZ
+);
+
+CREATE INDEX IF NOT EXISTS idx_calls_deleted_at ON calls(deleted_at);
+CREATE INDEX IF NOT EXISTS idx_calls_account_id ON calls(account_id);
+CREATE INDEX IF NOT EXISTS idx_calls_inbox_id ON calls(inbox_id);
+CREATE INDEX IF NOT EXISTS idx_calls_conversation_id ON calls(conversation_id);
+CREATE INDEX IF NOT EXISTS idx_calls_contact_id ON calls(contact_id);
+CREATE INDEX IF NOT EXISTS idx_calls_message_id ON calls(message_id);
+CREATE INDEX IF NOT EXISTS idx_calls_accepted_by_agent_id ON calls(accepted_by_agent_id);
+CREATE INDEX IF NOT EXISTS idx_calls_provider_call_id ON calls(provider, provider_call_id);
+
+ALTER TABLE calls ADD COLUMN IF NOT EXISTS started_at TIMESTAMPTZ;
+ALTER TABLE calls ADD COLUMN IF NOT EXISTS end_reason VARCHAR(255);