Relates to CW-1286


Adds a Seer project single-repo endpoint **`ProjectSeerRepoEndpoint`**
(`GET`/`PUT`/`DELETE`) with ProjectPermission.

- **GET**: Returns the Seer project repo config (branch name,
instructions, branch overrides) for a given repository. 404 if not
connected, inactive, or unsupported provider.
- **PUT**: Partial update — only updates fields present in the request
body. Uses `select_for_update` on the SeerProjectRepository row.
Replaces branch overrides if provided; preserves them if omitted. 404 if
not found.
- **DELETE**: Removes the Seer project repo. Branch overrides
cascade-delete. 404 if not found.

All three methods filter by supported SCM providers
(`get_supported_scm_providers`), so repos with unsupported providers
return 404.

---------

Co-authored-by: getsantry[bot] <66042841+getsantry[bot]@users.noreply.github.com>

Fixes CW-1286

Depends on #115199 and #115904

Adds a bulk Seer project repos endpoint **`ProjectSeerReposEndpoint`**
(`GET`/`POST`/`PUT`) at `/api/0/projects/{org}/{project}/seer/repos/`
with `ProjectPermission`.

### GET — List connected repos

Lists all connected Seer project repos for a project. Supports search by
`name` (substring, case-insensitive) and `provider` (exact match,
supports `=`, `!=`, `IN`, `NOT IN`, with or without `integrations:`
prefix), sorting by `name`/`-name`/`provider`/`-provider`, and offset
pagination. Excludes inactive repos and unsupported providers.

```
GET /api/0/projects/my-org/my-project/seer/repos/?query=relay&sortBy=-name
```

### POST — Add repos

Upserts repos by ID. Validates all repo IDs are active, belong to the
org, and have supported providers — returns 400 with invalid IDs if not.
Rejects empty lists, duplicate repo IDs, and duplicate branch overrides.

```
POST /api/0/projects/my-org/my-project/seer/repos/
{
  "repos": [
    {"repositoryId": 1, "branchName": "main", "instructions": "use pytest"},
    {"repositoryId": 2, "branchOverrides": [
      {"tagName": "environment", "tagValue": "production", "branchName": "release"}
    ]}
  ]
}
```

### PUT — Replace all repos

Replaces all connected repos for the project atomically. Accepts an
empty list to disconnect all repos. Same validation as POST for
non-empty lists.

```
PUT /api/0/projects/my-org/my-project/seer/repos/
{
  "repos": [
    {"repositoryId": 3, "branchName": "develop"}
  ]
}
```

---------

Co-authored-by: getsantry[bot] <66042841+getsantry[bot]@users.noreply.github.com>