Increase OAuth connection URL column lengths beyond 512 characters

[exhuma]

Problem

The oauth_connection table defines image_url and profile_url as Unicode(512), capping both fields at 512 characters. OAuth providers (e.g. Google, GitHub) can return image and profile URLs that exceed this limit, causing silent truncation or insertion errors.

Affected locations

Field	File	Line	Current constraint
profile_url	backend/src/powonline/model.py	405	Unicode(512)
image_url	backend/src/powonline/model.py	406	Unicode(512)
profile_url	backend/alembic/versions/5be3d628dcc3_lost_tracker_alignment.py	116	Unicode(512)
image_url	backend/alembic/versions/5be3d628dcc3_lost_tracker_alignment.py	117	Unicode(512)

The image_url field is also written in backend/src/powonline/core.py:884 inside by_social_connection() when creating a new connection after a successful OAuth login.

Proposed fix

1. ORM model (model.py)

Change both column definitions to unbounded Unicode (or a very large limit such as Unicode(2048)):

# before
profile_url: Mapped[str | None] = mapped_column(Unicode(512))
image_url:   Mapped[str | None] = mapped_column(Unicode(512))

# after
profile_url: Mapped[str | None] = mapped_column(Unicode)
image_url:   Mapped[str | None] = mapped_column(Unicode)

Using an unbounded Unicode column maps to TEXT in PostgreSQL, which has no practical length limit and is the safest choice for arbitrary URLs.

2. New Alembic migration

Create a new migration that uses ALTER COLUMN to remove the length restriction:

def upgrade() -> None:
    op.alter_column("oauth_connection", "profile_url",
                    type_=sa.Unicode(), existing_type=sa.Unicode(512))
    op.alter_column("oauth_connection", "image_url",
                    type_=sa.Unicode(), existing_type=sa.Unicode(512))

def downgrade() -> None:
    op.alter_column("oauth_connection", "image_url",
                    type_=sa.Unicode(512), existing_type=sa.Unicode())
    op.alter_column("oauth_connection", "profile_url",
                    type_=sa.Unicode(512), existing_type=sa.Unicode())

Note: Do not modify the existing 5be3d628dcc3 migration — that would break environments that have already applied it.

Acceptance criteria

• Both image_url and profile_url columns accept URLs longer than 512 characters without error or truncation.
• A new Alembic migration exists and applies cleanly (alembic upgrade head).
• model.py reflects the updated column types.
• Existing tests continue to pass (pytest).