Add file visibility (PRIVATE/PUBLIC) + public files API

Adds a visibility enum to files with PRIVATE (default) and PUBLIC states.
PUBLIC files are accessible via an unauthenticated /api/files/v1/{fileID}
endpoint that redirects to a presigned S3 URL. Introduces pkg/file service
to manage file operations. Logo uploads (trust centers, organizations,
frameworks, references) are marked PUBLIC; other files are PRIVATE.
Includes database migration and backfill for existing logos.

Signed-off-by: Bryan Frimin <bryan@getprobo.com>

pkg/coredata/migrations/20260318T120000Z.sql

@@ -0,0 +1,41 @@
+-- Copyright (c) 2026 Probo Inc <hello@getprobo.com>.
+--
+-- Permission to use, copy, modify, and/or distribute this software for any
+-- purpose with or without fee is hereby granted, provided that the above
+-- copyright notice and this permission notice appear in all copies.
+--
+-- THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+-- REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+-- AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+-- INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+-- LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+-- OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+-- PERFORMANCE OF THIS SOFTWARE.
+
+ALTER TABLE files ADD COLUMN visibility TEXT NOT NULL DEFAULT 'PRIVATE';
+
+-- Backfill trust center logos to PUBLIC
+UPDATE files SET visibility = 'PUBLIC'
+WHERE id IN (
+    SELECT logo_file_id FROM trust_centers WHERE logo_file_id IS NOT NULL
+    UNION
+    SELECT dark_logo_file_id FROM trust_centers WHERE dark_logo_file_id IS NOT NULL
+);
+
+-- Backfill organization logos to PUBLIC
+UPDATE files SET visibility = 'PUBLIC'
+WHERE id IN (
+    SELECT logo_file_id FROM organizations WHERE logo_file_id IS NOT NULL
+    UNION
+    SELECT horizontal_logo_file_id FROM organizations WHERE horizontal_logo_file_id IS NOT NULL
+);
+
+-- Backfill framework logos to PUBLIC
+UPDATE files SET visibility = 'PUBLIC'
+WHERE id IN (
+    SELECT light_logo_file_id FROM frameworks WHERE light_logo_file_id IS NOT NULL
+    UNION
+    SELECT dark_logo_file_id FROM frameworks WHERE dark_logo_file_id IS NOT NULL
+);
+
+ALTER TABLE files ALTER COLUMN visibility DROP DEFAULT;