feat(profile): add round avatar with upload and cropping

- backend: user_avatar_objects binding (PG+SQLite), GC preserves avatar blobs,
  avatar service (EXIF-stripping WebP 128/512), public avatar read route
- web: avatar upload/crop on profile, round avatar in header/members/comments/author
- android: avatar upload/crop, round avatar on profile and albums header
- openspec: add-user-avatar proposal/design/specs/tasks

backend/migrations/000014_user_avatar_objects.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS user_avatar_objects;

backend/migrations/000014_user_avatar_objects.up.sql

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+-- A user's avatar is stored as journaled objects bound by role, mirroring
+-- photo_objects. Keeping the binding in the ledger means garbage collection
+-- preserves avatar blobs while they are referenced and reclaims them once the
+-- avatar is replaced or removed.
+CREATE TABLE IF NOT EXISTS user_avatar_objects (
+    user_id   text NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    object_id text NOT NULL REFERENCES objects(id) ON DELETE RESTRICT,
+    role      text NOT NULL CHECK (role IN ('avatar', 'avatar_small')),
+    PRIMARY KEY (user_id, role)
+);
+
+CREATE INDEX IF NOT EXISTS user_avatar_objects_object_idx ON user_avatar_objects (object_id);

backend/migrations/sqlite/000006_user_avatar_objects.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS user_avatar_objects;

backend/migrations/sqlite/000006_user_avatar_objects.up.sql

@@ -0,0 +1,12 @@
+-- A user's avatar is stored as journaled objects bound by role, mirroring
+-- photo_objects. Keeping the binding in the ledger means garbage collection
+-- preserves avatar blobs while they are referenced and reclaims them once the
+-- avatar is replaced or removed.
+CREATE TABLE user_avatar_objects (
+    user_id   TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    object_id TEXT NOT NULL REFERENCES objects(id) ON DELETE RESTRICT,
+    role      TEXT NOT NULL CHECK (role IN ('avatar', 'avatar_small')),
+    PRIMARY KEY (user_id, role)
+);
+
+CREATE INDEX IF NOT EXISTS user_avatar_objects_object_idx ON user_avatar_objects (object_id);