feat(backend): add user custom attributes system

Add a flexible user attribute system that allows admins to define
custom fields for users (text, textarea, number, email, url, date,
select, multi_select types).

- Add Ent schemas for UserAttributeDefinition and UserAttributeValue
- Add service layer with validation logic
- Add repository layer with batch operations support
- Add admin API endpoints for CRUD and reorder operations
- Add batch API for loading attribute values for multiple users
- Add database migration (018_user_attributes.sql)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/migrations/018_user_attributes.sql

@@ -0,0 +1,48 @@
+-- Add user attribute definitions and values tables for custom user attributes.
+
+-- User Attribute Definitions table (with soft delete support)
+CREATE TABLE IF NOT EXISTS user_attribute_definitions (
+    id              BIGSERIAL PRIMARY KEY,
+    key             VARCHAR(100) NOT NULL,
+    name            VARCHAR(255) NOT NULL,
+    description     TEXT DEFAULT '',
+    type            VARCHAR(20) NOT NULL,
+    options         JSONB DEFAULT '[]'::jsonb,
+    required        BOOLEAN NOT NULL DEFAULT FALSE,
+    validation      JSONB DEFAULT '{}'::jsonb,
+    placeholder     VARCHAR(255) DEFAULT '',
+    display_order   INT NOT NULL DEFAULT 0,
+    enabled         BOOLEAN NOT NULL DEFAULT TRUE,
+    created_at      TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at      TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    deleted_at      TIMESTAMPTZ
+);
+
+-- Partial unique index for key (only for non-deleted records)
+-- Allows reusing keys after soft delete
+CREATE UNIQUE INDEX IF NOT EXISTS idx_user_attribute_definitions_key_unique
+    ON user_attribute_definitions(key) WHERE deleted_at IS NULL;
+
+CREATE INDEX IF NOT EXISTS idx_user_attribute_definitions_enabled
+    ON user_attribute_definitions(enabled);
+CREATE INDEX IF NOT EXISTS idx_user_attribute_definitions_display_order
+    ON user_attribute_definitions(display_order);
+CREATE INDEX IF NOT EXISTS idx_user_attribute_definitions_deleted_at
+    ON user_attribute_definitions(deleted_at);
+
+-- User Attribute Values table (hard delete only, no deleted_at)
+CREATE TABLE IF NOT EXISTS user_attribute_values (
+    id              BIGSERIAL PRIMARY KEY,
+    user_id         BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    attribute_id    BIGINT NOT NULL REFERENCES user_attribute_definitions(id) ON DELETE CASCADE,
+    value           TEXT DEFAULT '',
+    created_at      TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at      TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+
+    UNIQUE(user_id, attribute_id)
+);
+
+CREATE INDEX IF NOT EXISTS idx_user_attribute_values_user_id
+    ON user_attribute_values(user_id);
+CREATE INDEX IF NOT EXISTS idx_user_attribute_values_attribute_id
+    ON user_attribute_values(attribute_id);