feat: add image management functionality

- Implemented image folder repository with CRUD operations.
- Added image reference repository for managing image references to articles.
- Created image repository for handling image assets, including listing, inserting, updating, and deleting images.
- Introduced image usage repository to track storage usage and quotas for tenants.
- Added SQL queries for image assets, folders, references, and usage.
- Developed image handler for HTTP endpoints to manage images and folders.
- Created database migration scripts for image-related tables and structures.

server/migrations/20260415100000_create_image_tables.down.sql

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+DROP TABLE IF EXISTS image_asset_references;
+DROP TABLE IF EXISTS image_assets;
+DROP TABLE IF EXISTS image_folders;
+DROP TABLE IF EXISTS tenant_image_storage_usage;
+
+UPDATE plans
+SET quota_policy_json = quota_policy_json - 'image_storage_bytes'
+WHERE deleted_at IS NULL;

server/migrations/20260415100000_create_image_tables.up.sql

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+CREATE EXTENSION IF NOT EXISTS pg_trgm;
+
+CREATE TABLE image_folders (
+  id         BIGSERIAL PRIMARY KEY,
+  tenant_id  BIGINT NOT NULL,
+  name       VARCHAR(100) NOT NULL,
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  CONSTRAINT uq_image_folders_tenant_name UNIQUE (tenant_id, name),
+  CONSTRAINT uq_image_folders_tenant_id UNIQUE (tenant_id, id)
+);
+
+CREATE INDEX idx_image_folders_tenant ON image_folders (tenant_id);
+
+CREATE TABLE image_assets (
+  id           BIGSERIAL PRIMARY KEY,
+  tenant_id    BIGINT NOT NULL,
+  folder_id    BIGINT,
+  object_key   TEXT NOT NULL UNIQUE,
+  name         VARCHAR(255) NOT NULL,
+  size_bytes   BIGINT NOT NULL,
+  status       VARCHAR(20) NOT NULL DEFAULT 'active',
+  created_by   BIGINT NOT NULL,
+  updated_at   TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  created_at   TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  deleted_at   TIMESTAMPTZ,
+  CONSTRAINT fk_image_assets_folder
+    FOREIGN KEY (tenant_id, folder_id)
+    REFERENCES image_folders(tenant_id, id),
+  CONSTRAINT uq_image_assets_tenant_id UNIQUE (tenant_id, id)
+);
+
+CREATE INDEX idx_image_assets_tenant_folder ON image_assets (tenant_id, folder_id) WHERE status = 'active' AND deleted_at IS NULL;
+CREATE INDEX idx_image_assets_tenant_created ON image_assets (tenant_id, created_at DESC) WHERE status = 'active' AND deleted_at IS NULL;
+CREATE INDEX idx_image_assets_tenant_status ON image_assets (tenant_id, status, created_at DESC);
+CREATE INDEX idx_image_assets_name_trgm ON image_assets USING GIN (name gin_trgm_ops) WHERE status = 'active' AND deleted_at IS NULL;
+
+CREATE TABLE image_asset_references (
+  id             BIGSERIAL PRIMARY KEY,
+  tenant_id      BIGINT NOT NULL,
+  image_asset_id BIGINT NOT NULL,
+  article_id     BIGINT NOT NULL,
+  ref_scope      VARCHAR(20) NOT NULL,
+  updated_at     TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  created_at     TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+  CONSTRAINT fk_image_asset_references_asset
+    FOREIGN KEY (tenant_id, image_asset_id)
+    REFERENCES image_assets(tenant_id, id)
+    ON DELETE CASCADE,
+  CONSTRAINT uq_image_asset_reference UNIQUE (image_asset_id, article_id, ref_scope)
+);
+
+CREATE INDEX idx_image_asset_references_asset ON image_asset_references (tenant_id, image_asset_id);
+CREATE INDEX idx_image_asset_references_article ON image_asset_references (tenant_id, article_id);
+
+CREATE TABLE tenant_image_storage_usage (
+  tenant_id   BIGINT PRIMARY KEY,
+  used_bytes  BIGINT NOT NULL DEFAULT 0,
+  updated_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+UPDATE plans
+SET quota_policy_json = quota_policy_json ||
+  CASE plan_code
+    WHEN 'free' THEN '{"image_storage_bytes":104857600}'::jsonb
+    WHEN 'regular' THEN '{"image_storage_bytes":1073741824}'::jsonb
+    WHEN 'premium' THEN '{"image_storage_bytes":2147483648}'::jsonb
+    ELSE '{}'::jsonb
+  END
+WHERE deleted_at IS NULL;

server/migrations/20260416113000_add_image_asset_content_hash.down.sql

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+DROP INDEX IF EXISTS idx_image_assets_tenant_content_hash_active;
+
+ALTER TABLE image_assets
+DROP COLUMN IF EXISTS content_hash;

server/migrations/20260416113000_add_image_asset_content_hash.up.sql

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+ALTER TABLE image_assets
+ADD COLUMN content_hash VARCHAR(64);
+
+CREATE INDEX idx_image_assets_tenant_content_hash_active
+ON image_assets (tenant_id, content_hash)
+WHERE content_hash IS NOT NULL
+  AND status = 'active'
+  AND deleted_at IS NULL;