feat: table migration

Signed-off-by: Mouad BANI <mouad-mb@outlook.com>

Author: mbani01

backend/src/osspckgs/migrations/V1784905809__repo_well_known_files.sql

@@ -0,0 +1,22 @@
+-- Well-known files inventory per repo, populated by the github-repos-enricher.
+CREATE TABLE IF NOT EXISTS repo_well_known_files (
+    id bigserial PRIMARY KEY,
+    repo_id bigint NOT NULL REFERENCES repos (id) ON DELETE CASCADE,
+    file_type text NOT NULL, -- security | contributing | governance | maintainers | code_of_conduct | readme
+    directory text NOT NULL, -- root | .github | docs
+    path text NOT NULL,
+    blob_oid text NOT NULL,
+    first_seen_at timestamptz NOT NULL DEFAULT NOW(),
+    -- observation time, not commit time; bumped on content change, disappearance, reappearance
+    change_detected_at timestamptz NOT NULL DEFAULT NOW(),
+    checked_at timestamptz NOT NULL,
+    deleted_at timestamptz,
+    UNIQUE (repo_id, path)
+);
+
+CREATE INDEX IF NOT EXISTS repo_well_known_files_change_detected_at_idx
+    ON repo_well_known_files (change_detected_at);
+
+CREATE INDEX IF NOT EXISTS repo_well_known_files_file_type_idx
+    ON repo_well_known_files (file_type)
+    WHERE deleted_at IS NULL;