feat: implement CMS for content management (WEB-02/03/04/06)

Introduces Filament management for site settings, services, portfolio cases, and testimonials with admin-only policies, validated media uploads, and deterministic seeding. Includes comprehensive design and task documentation for the implementation.

Co-authored-by: Cursor <cursoragent@cursor.com>

openspec/changes/archive/2026-07-29-build-content-cms/specs/site-settings/spec.md

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+## ADDED Requirements
+
+### Requirement: Site settings singleton is manageable by admin only
+
+The system SHALL persist site-wide settings in a `site_settings` table as a typed singleton (SPEC WEB-06, §8.2). Fields MUST include brand name, hero copy (eyebrow, title, subtitle, CTA label), about summary, contact email/phone/city, social links (jsonb), default meta title/description, default OG image path and alt text, and optional analytics fields disabled by default.
+
+#### Scenario: Admin updates site settings
+
+- **WHEN** an admin saves the site settings form in Filament
+- **THEN** the singleton record is updated
+- **AND** labels and validation messages are in pt-BR
+
+#### Scenario: Assistant cannot access site settings
+
+- **WHEN** an assistant navigates to site settings in Filament
+- **THEN** access MUST be denied with HTTP 403
+
+#### Scenario: Default OG image requires alt text
+
+- **WHEN** an admin uploads a default OG image without alt text
+- **THEN** validation MUST fail with a pt-BR error message
+- **AND** alt text MUST remain optional when no default OG image is present
+
+#### Scenario: Singleton avoids generic key-value store
+
+- **WHEN** site settings are stored
+- **THEN** the system MUST use typed columns on `site_settings`
+- **AND** MUST NOT introduce a generic key/value configuration table