refactor: engineering improvements -- API versioning, structured logging, Alembic, error standardization, test coverage

- API versioning: all REST endpoints prefixed with /api/v1/
- Structured logging: replaced stdlib logging with structlog (console/JSON modes)
- Alembic migrations: versioned DB schema with initial migration
- Error standardization: global exception handlers for consistent envelope format
- Interrupt cleanup: asyncio background task for expired interrupt removal
- Integration tests: +30 tests (analytics, replay, openapi, error, session APIs)
- Frontend tests: +57 tests (all components, pages, useWebSocket hook)
- Backend: 557 tests, 89.75% coverage | Frontend: 80 tests, 16 test files

backend/alembic/README

@@ -0,0 +1 @@
+Generic single-database configuration.

backend/alembic/env.py

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+"""Alembic environment configuration for smart-support."""
+
+from __future__ import annotations
+
+import os
+from logging.config import fileConfig
+
+from sqlalchemy import engine_from_config, pool
+
+from alembic import context
+
+config = context.config
+
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+# No SQLAlchemy ORM models -- we use raw DDL migrations
+target_metadata = None
+
+
+def _get_url() -> str:
+    """Read DATABASE_URL from environment, falling back to alembic.ini."""
+    return os.environ.get("DATABASE_URL", "") or config.get_main_option(
+        "sqlalchemy.url", ""
+    )
+
+
+def run_migrations_offline() -> None:
+    """Run migrations in 'offline' mode.
+
+    Configures the context with just a URL so that an Engine
+    is not required.
+    """
+    url = _get_url()
+    context.configure(
+        url=url,
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def run_migrations_online() -> None:
+    """Run migrations in 'online' mode with a live database connection."""
+    configuration = config.get_section(config.config_ini_section, {})
+    configuration["sqlalchemy.url"] = _get_url()
+
+    connectable = engine_from_config(
+        configuration,
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+
+    with connectable.connect() as connection:
+        context.configure(connection=connection, target_metadata=target_metadata)
+
+        with context.begin_transaction():
+            context.run_migrations()
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()

backend/alembic/script.py.mako

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+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, Sequence[str], None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    ${downgrades if downgrades else "pass"}

backend/alembic/versions/001_initial_schema.py

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+"""Initial schema -- all application tables.
+
+Revision ID: a1b2c3d4e5f6
+Revises:
+Create Date: 2026-04-06
+"""
+
+from __future__ import annotations
+
+from alembic import op
+
+revision: str = "a1b2c3d4e5f6"
+down_revision: str | None = None
+branch_labels: tuple[str, ...] | None = None
+depends_on: tuple[str, ...] | None = None
+
+
+def upgrade() -> None:
+    op.execute(
+        """
+        CREATE TABLE IF NOT EXISTS conversations (
+            thread_id TEXT PRIMARY KEY,
+            created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+            last_activity TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+            total_tokens INTEGER NOT NULL DEFAULT 0,
+            total_cost_usd DOUBLE PRECISION NOT NULL DEFAULT 0.0,
+            status TEXT NOT NULL DEFAULT 'active'
+        )
+        """
+    )
+
+    op.execute(
+        """
+        CREATE TABLE IF NOT EXISTS active_interrupts (
+            interrupt_id TEXT PRIMARY KEY,
+            thread_id TEXT NOT NULL REFERENCES conversations(thread_id),
+            action TEXT NOT NULL,
+            params JSONB NOT NULL DEFAULT '{}',
+            created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+            resolved_at TIMESTAMPTZ,
+            resolution TEXT
+        )
+        """
+    )
+
+    op.execute(
+        """
+        CREATE TABLE IF NOT EXISTS sessions (
+            thread_id TEXT PRIMARY KEY,
+            last_activity TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+            has_pending_interrupt BOOLEAN NOT NULL DEFAULT FALSE,
+            created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+        )
+        """
+    )
+
+    op.execute(
+        """
+        CREATE TABLE IF NOT EXISTS analytics_events (
+            id BIGSERIAL PRIMARY KEY,
+            thread_id TEXT NOT NULL,
+            event_type TEXT NOT NULL,
+            agent_name TEXT,
+            tool_name TEXT,
+            tokens_used INTEGER NOT NULL DEFAULT 0,
+            cost_usd DOUBLE PRECISION NOT NULL DEFAULT 0.0,
+            duration_ms INTEGER,
+            success BOOLEAN,
+            error_message TEXT,
+            metadata JSONB NOT NULL DEFAULT '{}',
+            created_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+        )
+        """
+    )
+
+    # Migration columns added in Phase 4
+    op.execute(
+        """
+        ALTER TABLE conversations
+            ADD COLUMN IF NOT EXISTS resolution_type TEXT,
+            ADD COLUMN IF NOT EXISTS agents_used TEXT[],
+            ADD COLUMN IF NOT EXISTS turn_count INTEGER NOT NULL DEFAULT 0,
+            ADD COLUMN IF NOT EXISTS ended_at TIMESTAMPTZ
+        """
+    )
+
+
+def downgrade() -> None:
+    op.execute("DROP TABLE IF EXISTS analytics_events")
+    op.execute("DROP TABLE IF EXISTS sessions")
+    op.execute("DROP TABLE IF EXISTS active_interrupts")
+    op.execute("DROP TABLE IF EXISTS conversations")